There is no user-auth or user-auth API on Webflow, as it’s only content-based.
See ► [LIST] Feature Availability & Limits - Project settings - Forum | Webflow
I suggest you complete the design, then export and hand over to your web developer to integrate into their sharepoint.
Example:
http://healthxchange.webflow.io (Webflow) 
https://www.healthxchange.sg (Sharepoint)