This article shows how to constraint the customer link field using an Entity like an Account for External Contacts or an Organizational Unit for internal employees.
Please follow the instructions below for constraint the customer based on the Entity selected, an Account for the External Contacts or an Organizational Unit for using internal Employees:
1.- In the Incident object, ensure that the EntityLink field is configured with the relationship:
In that relationship, by default it is possible to add any type of Entity, but we can filter it. In this case, we are going to use just Account (for ExternalContacts) and Organizational Units (for Employees)
2.- Add the field EntityLink to the IncidentHeader form (or whichever used for the header)
3.- Finally, modify the relationship IncidentAssociatedCustomer adding the condition:
$([OtherObject]EntityLink_RecID == EntityLink_RecID)