Signature Field Box not exporting on InDesign Interactive PDF

When I export a document as an InDesign Interactive PDF, the text fields show in blue, but the signature field is not there. How can I export the interactive document with the signature field available to click into and edit?

This is how the fields look on InDesign:

However this is the document when exported as an interactive doc:

Our clients are older, so the aim is to make our documents as easy as possible to edit!

Thanks in advance for any help. 🙂