diff --git a/README.md b/README.md
index 1e49760..89121b7 100644
--- a/README.md
+++ b/README.md
@@ -191,6 +191,14 @@ const provider = createPkceProvider<Account>({
     authorizeUrl: ({ handshake }) => `${handshake.baseUrl as string}/oauth/authorize`,
     tokenUrl: ({ handshake }) => `${handshake.baseUrl as string}/oauth/token`,
     clientId: ({ flags }) => flags.clientId as string,
+    // Optional: add extra form parameters to the authorization-code token
+    // request (e.g. Zendesk's `expires_in` / `refresh_token_expires_in`).
+    tokenRequestParams: async ({ handshake, flags }) => ({
+        // `handshake` carries the authorize-time state; `flags` carries any
+        // runtime CLI flags that shaped the flow.
+        expires_in: 172800,
+        refresh_token_expires_in: 7776000,
+    }),
     validate: async ({ token, handshake }) => probeUser(token, handshake.baseUrl as string),
 })
 
@@ -212,7 +220,9 @@ attachLoginCommand<Account>(auth, {
 
 `attachLoginCommand` returns the new `Command` so you can chain `.description(...)` / `.option(...)` / `.addHelpText(...)`. Any consumer-attached options land in the `flags` object passed to `resolveScopes`, `onSuccess`, and the provider hooks.
 
-The `authorizeUrl` / `tokenUrl` / `clientId` resolvers may return `string` **or** `Promise<string>` — so a consumer can resolve the base URL or client id asynchronously (reading config, prompting the user) without abandoning `createPkceProvider`. An injected `fetchImpl` is used for the token exchange **and** the refresh grant (threaded into `oauth4webapi` via its `customFetch`), so a custom transport — proxy dispatcher, decompression — applies on every OAuth call rather than being bypassed by the library's global `fetch`.
+The `authorizeUrl` / `tokenUrl` / `clientId` resolvers may return `string` **or** `Promise<string>` — so a consumer can resolve the base URL or client id asynchronously (reading config, prompting the user) without abandoning `createPkceProvider`. `tokenRequestParams` is the equivalent escape hatch for authorization-code token requests: it is optional, receives the same `handshake` + `flags` context as the other provider hooks, and may return either a plain object or a `Promise` when the extra parameters need to be resolved asynchronously. This is useful for providers that require non-standard form fields such as Zendesk's `expires_in` or `refresh_token_expires_in`.
+
+An injected `fetchImpl` is used for the token exchange **and** the refresh grant (threaded into `oauth4webapi` via its `customFetch`), so a custom transport — proxy dispatcher, decompression — applies on every OAuth call rather than being bypassed by the library's global `fetch`.
 
 #### Quick start (Dynamic Client Registration)
 
diff --git a/src/auth/providers/pkce.test.ts b/src/auth/providers/pkce.test.ts
index 6afc3fa..bd7bd5a 100644
--- a/src/auth/providers/pkce.test.ts
+++ b/src/auth/providers/pkce.test.ts
@@ -81,6 +81,34 @@ describe('createPkceProvider', () => {
         expect(url.searchParams.get('client_id')).toBe('async-client')
     })
 
+    it('adds provider-defined token endpoint parameters (e.g. Zendesk max expiry values)', async () => {
+        const fetchImpl = vi.fn(async (_input: RequestInfo | URL, init: RequestInit = {}) => {
+            const body = new URLSearchParams(init.body as string)
+            expect(body.get('expires_in')).toBe('172800')
+            expect(body.get('refresh_token_expires_in')).toBe('7776000')
+            return respond({ access_token: 'tok-1', expires_in: 3600 })
+        }) as unknown as typeof fetch
+
+        const provider = createPkceProvider<Account>({
+            authorizeUrl: 'https://example.com/oauth/authorize',
+            tokenUrl: 'https://example.com/oauth/token',
+            clientId: 'client-xyz',
+            tokenRequestParams: () => ({
+                expires_in: 172800,
+                refresh_token_expires_in: 7776000,
+            }),
+            validate,
+            fetchImpl,
+        })
+
+        await provider.exchangeCode({
+            code: 'the-code',
+            state: 's',
+            redirectUri: 'http://localhost/callback',
+            handshake: { codeVerifier: 'the-verifier', clientId: 'client-xyz' },
+        })
+    })
+
     it('exchangeCode POSTs without client_secret and surfaces token endpoint failures as AUTH_TOKEN_EXCHANGE_FAILED', async () => {
         const ok = createPkceProvider<Account>({
             authorizeUrl: 'unused',
diff --git a/src/auth/providers/pkce.ts b/src/auth/providers/pkce.ts
index 33d7f55..6441316 100644
--- a/src/auth/providers/pkce.ts
+++ b/src/auth/providers/pkce.ts
@@ -49,6 +49,13 @@ export type PkceProviderOptions<TAccount extends AuthAccount = AuthAccount> = {
     tokenUrl: PkceLazyString
     /** Pre-registered client_id, or a function that derives one from `input.flags`. */
     clientId: PkceLazyString
+    /** Additional form-encoded parameters to include in the token request body. */
+    tokenRequestParams?: (ctx: {
+        handshake: Record<string, unknown>
+        flags: Record<string, unknown>
+    }) =>
+        | Record<string, string | number | undefined>
+        | Promise<Record<string, string | number | undefined>>
     /** How to join scopes in the authorize URL. Default `' '` (RFC 6749). Pass `','` for Todoist. */
     scopeSeparator?: string
     verifierAlphabet?: string
@@ -130,12 +137,21 @@ export function createPkceProvider<TAccount extends AuthAccount>(
             const flags = (input.handshake.flags as Record<string, unknown> | undefined) ?? {}
             const tokenUrl = await resolve(options.tokenUrl, input.handshake, flags)
 
+            const extraTokenParams = await (options.tokenRequestParams?.({
+                handshake: input.handshake,
+                flags,
+            }) ?? {})
             const body = new URLSearchParams({
                 grant_type: 'authorization_code',
                 code: input.code,
                 redirect_uri: input.redirectUri,
                 client_id: clientId,
                 code_verifier: verifier,
+                ...Object.fromEntries(
+                    Object.entries(extraTokenParams)
+                        .filter(([, value]) => value !== undefined)
+                        .map(([key, value]) => [key, String(value)]),
+                ),
             })
 
             const result = await postTokenEndpoint({