Can I create a document using a template and prefilling fields with signNow's API?

Yes. Create a template by flattening an existing document.

{"document_name":"my template", "document_id":"8a8fb421a75a605625d273045a687e588628e43c"}

To prefill documents with a customer's data (i.e. from a database) in signNow, you can use smart fields. Use this link to learn how to add smart fields. To prefill smart fields via an API, use this call