How to Add Sign Field in PDF File by using Microsoft Power Automate.

there's no Sign connector for this. You would need to make use of the premium http connector and make calls to

get /agreements/{agreementId}/formFields and 

put /agreements/{agreementId}/formFields

though it's easier to ensure fields are already on your files. if there's a master template of the file, add Sign fields to those by either using text tags or acrobat fields:

https://helpx.adobe.com/uk/sign/using/create-forms.html