To do what you want, @Marius_Scholtz, add an expression to the
Initial value field of the column of the service call that references data from your customer table. For example…
Suppose the Customers table has the (hopefully self-explanatory) columns: [CustID], [Name], [Phone], and [Address].
Suppose the ServiceCalls table has the columns: [Customer] (Ref to Customers table); [SvcName] (Name of contact for service call; [SvcAddress] (Address for service call); and [SvcPhone] (Phone for service call).
The [Customer] column of the ServiceCalls table would be a Ref type with a ReferencedTableName of Customers. The Valid_If field could be set to =orderby(Customers[CustID], [Name]) to provide a drop-down menu of customers sorted by their names.
The [SvcName] column would be a Name type with an Initial value of =lookup([_ThisRow].[Customer], Customers, CustID, Name), which will fill-in the value of the customer’s [Name] column when a customer is chosen. The filled-in name will then be editable by the app user.
Likewise, the [SvcAddress] column, with Address type, would have an Initial value of =lookup([_ThisRow].[Customer], Customers, CustID, Address) to pre-fill the customer’s address.
And similarly, [SvcPhone]'s Initial value would be =lookup([_ThisRow].[Customer], Customers, CustID, Phone).
Note in the above we use the Initial value field, not the App formula field.