How to Add Default image to In-Line image field or to Stamp field in Adobe Sign using the Adobe Sign APIs?

Question

I am trying to add a default image like a stamp to my transient document using Adobe Sign form fields API. In Chat GPT and Gemini, it is mentioned to send the base64 encoded string of the image to the Image/Stamp field in Document via the Adobe Sign API but it didn't work.

Even if not through API, can we add default image to Image/Stamp field using templates in Adobe Sign Console itself.

PUT -- /agreements/{agreementId}/formFields/mergeInfo

I tried using this endpoint in Adobe Sign API to add a default image/stamp to the field in the document. I sent the field name and the image as base64 encoded string.