Need primary flag to default when a new value is added via REST api

Summary

We need the national identifier primary flag to be set when a new value is sent by REST api.

Content

One of our third party providers is updating the National Identifier via REST. However, they cannot update the primary flag associated with that particular national id. This causes an issue with our matching process between cloud and on-prem. On-prem is looking for this flag, if it doesn't find one it moves on and as a result we get duplicate users in our system. 

Is there any way to setup an automatic default of this flag when a new value is entered via REST api?