The relationship is not a factor here as your script avoids using any relationship.
Copy and Paste have several drawbacks and there are better alternatives. One draw back you have identified. If the referenced field is not on the layout and the cursor is not in the field (Paste can select the field for you), the step fails to work. Another draw back is that the copy step replaces any data the user may have copied with the data you are moving from one table to another and this can confuse and irritate your users.
Use this script instead:
#must be performed on layout referring to tblNetworkInitiatives
Set Variable [$ID ; Value: tblNetworkInitiatives::niID ]
Go to Layout ["networkAdmin_Library_AddBudget" (tblNetworkINiativesBudget)]
Set Field [tblNetworkInitiativeBudget::niID ; $ID]
thanks heaps works brilliantly