Do I understand correctly that you want force the SENDER to populate some data fields prior to sending an Envelope? While the Sender MAY populate any of the data fields prior to sending an envelope, it's not possible to force them to do so. This is because each data field is "owned-by" / "assigned-to" a Recipient of the Envelope, and the (recipient) owner of the field is the person who will be required to populate it (if Required = true for the field).
As a work-around, you might consider specifying the Sender of the Envelope as the first recipient in the Envelope's workflow, and assiging the relevant (required) Data Field(s) to him/her.
Alternatively, you could build a web form to collect the (required) data from the Sender, then use the API to "Create/Send Envelope From Template" when they submit the web form (pre-populating the data fields with the API request).