I have an external web developer who is able to hit my NS Integration Endpoint with Postman, but when he runs the Access Tokens through his code, the call isn't going through. He keeps receiving 403 Error. I assigned him new Access Tokens but that didn't seem to help. Any other suggestions on how to trouble shoot this inside NS? (Assuming it's a NS problem)

your only choice is probably a Login Audit Trail Search

Thanks, I'll check it out.

otherwise you probably want your developer to figure out how to log what the http requests is

His code is probably not generating the correct signature.

A common scenario is that the code isn't sending "Realm" in the header. This should contain the account number with underscores instead of hyphens. 12345_SB1 for example