send domain name when calling Rest api


I am using OO Rest API to launch a flow externally.

As mentioned in OO docs, In the Basic Authentification i am sending: Authorization:Basic base64 (username:password), but i want to send also the domain which is in my case Internal.

eg: I have an internal  oo user : myInternalUser

when i call the API using this user,in the server log,  i get : 

( WARN - Failed to login user "enterpriseDomain\myInternalUser"

So i unsderstand from this, that OO by deault take the domain as enterpriseDomain instead of Internal.

Could someone tell me how to do change that?