Mandatory in RelatedRecordId in ServiceResource

I'm having a problem and I need SalesforceScheduler functionalities in my Education Cloud organization. Therefore, I'm going to register all the teachers in my organization as ServiceResource records. The problem is that not all teachers will have an active user within Salesforce, but rather they will enter through Experience Cloud login licenses. When registering records, it says:

But I've looked in both the layout and the object manager and this field is not mandatory. Where can I look at it?

Another thing that occurred to me is to fill it out through a trigger, always entering my user name. In this case, the problem is that it gives the following error:

What solutions do you see feasible for my problem?

Thanks in advance.