# callbacks.onSignatureRequest

Part of [Callbacks](https://kernstra.com/docs/web-sdk/config/callbacks) in the Kernstra Web SDK 1.0.0 configuration reference.

- Type: `(field: FormFieldInfo | null) => Promise<{ dataUrl: string } | null>`
- Page: https://kernstra.com/docs/web-sdk/config/callbacks/onsignaturerequest

Replaces the signature dialog, which then never opens. Called by the Sign tool (`field` is `null`) and by a form's signature field (`field` is that field); without the `extract` feature the field is blanked as `formFieldChanged`'s is. Must resolve with `{ dataUrl }` where `dataUrl` is a `data:` URL of a PNG, JPEG, WebP, GIF or BMP image, at most 10 MB. Anything else — `null`, a web address, SVG, a missing `dataUrl` — is a cancel, and so is a rejected promise (logged with `console.warn`). While a request is open, clicking the page or the same signature field again does not ask a second time. If the user switches tool before you answer, the answer is dropped.

Web page: https://kernstra.com/docs/web-sdk/config/callbacks/onsignaturerequest