Set value for field from another model when adding a row

So I am trying to set the value of a new row field via the save button. The value should be the Sales_Rep__r.Id from the object Opp, Saved to
Sales_Rep_1__c on the OppItem object.

From the documentation I though I just needed to call the value via {{$}} or Opp.Sales_rep_r.Id But when ever I try like below, nothing is passed to the new field.

Any ideas?

OppItem <p>{{Sales_Rep__r.Id}} should  {{$}}</p>

It seems we have been unable to do this reliably also. The approach I’ve been shown is to set Conditions on the OppItem model, for Sales_Rep_1__c and for OpportunityId. Set the value as “Field from another model” and reference Opp. (You could also make those conditions filterable and use the “Activate & Set value of Model Condition” action if you needed to do so conditionally.)

Hmmm Could you post an example,   I am trying that but not getting any results that way.

Looking for an example, the first instance I checked actually uses the method you proposed here. I wondered if there were any difference, I looked at your page again and saw: {{$}}. But the field name is Sales_Rep__c. See if case isn’t the case!