API for external text tags not wokring

im having issues with the API, previously i used sign request sandbox which worked great and my portal was pulling data and adding it to the agreements before sending, when trying to add the prefill tags its not pulling the data and then sending the agreement, basically box is refusing the prefill tags being added before sending the document, does anyone know if this is possible for the API to add text tags from my platform to be added to an agreement then be sent for signature ?