post https://ext.edusign.fr/v1/document/custom/send/
Send a document for signature to student(s). In order to add an admin signature in the document, provide an admin_signature image URL. Provide the templateId, students ID array, user_id of the school administration, and custom fields filled. If fields are missing, send a request without them; the response will be the array of fields. For admin signature in the document, provide an admin_signature image URL.