At the moment, if your agreement has any fields where the "Assigned To" is set to "Sender (prefill)" you cannot use the Adobe Sign APIs to send the document for signing. Instead, the document gets stuck in the "Waiting for you" area. If you change the "Assigned To" to something else and run the same API calls, Adobe Sign automatically sends out the signature requests. See the following example linked below:

https://powerusers.microsoft.com/t5/General-Power-Automate/Adobe-Sign-document-waiting-for-me-to-Prefill/m-p/297325/highlight/true#M28786

Like in the above-linked example, we have forms that are automatically distributed by a background process. If that fails, or for some reason another instance of that agreement needs to be manually initiated, having the "Sender (prefill)" is useful for manual Send-processes started in the Adobe Sign portal. The "Sender (prefill)" fields means that the sender will be prompted for the required fields before Adobe Sign sends out the signature requests.