There is currently no signature support on the online forms, but that's something I would like to add soon.

When generating a PDF via an API request, you can add an image field, and upload your user's signature as an image. You could also create a faux signature by using a text field with a handwriting font (The Dancing Script [1] font is available.)

This is a tool for developers to fill out PDFs, so in your own application you could use something like the signature_pad library [2], and then pass the signature images to FormAPI.

[1] https://fonts.google.com/specimen/Dancing+Script

[2] https://github.com/szimek/signature_pad