Using the Lookup Field property it is simple to indicate which column should be used from the lookup selected within the control. By default, the customString1 field(Primary) is used.
Click on Modules.
Select the Module with the appropriate form field.
Select the Forms tab.
Select the Field List button "Create/Update Fields".
Use the filter to find the relevant field.
Select Edit on the field.
You will need to enter the internal name of the form field column from the lookup selected in the Lookup Field property.
To get the Internal lookup column field name, navigate to the SharedLookup tab and find the shared lookup that you want to use.
Click on the Form button for the Sharedlookup you are going to use
Once in the shared lookup form click on the Configure this Field wrench for the filed you want to use in your lookup field.
Select the Debug tab to get the Internal Name for your shared lookup – Lookup Field.