Is it possible to embed where a field should go into a document?

Is it possible to embed where a field should go into a document?

I am using the Zoho Sign API to automate sending a file for signature. I am generating the base PDF in real-time and it can change based the content. I need the signature place to be on the last page in a specific location.

The "requests/[Request Id]/submit" endpoint allows me to add fields to the document when it gets sent. However, the problem is it's up to me to specify the x/y coord of where to place the signature block. Like I mentioned though, this can change - what I could much more easily do would be to embed into the source PDF document a code/text or something that would indicate where the signature should go.  Is there any support for a feature like this?