Question about custom fields API

I am having lots of trouble searching and finding solutions from DocuSign, Currently our Loan Origination System (we are financial Institution) does not send their mapped fields such as Account number, thru their API. I'm wondering if anyone here in the financial industry (bank or Credit Union) have any pointers on how to retrieve the LOS mapped field in order for us to file it in Cold Storage. If any financial institution is willing to assist, We would be glad to speak to you.

Thank you so much!