And should the script bring up the new record in that specific layout after creating it? (When a related record did not exist.)
If so, Say you have this relationship:
ParentTable::__pkParentID = Child::_fkParentID
and you enable "allow creation of records via this relaitonship" for the Child table.
Then This this script step:
Set Field [Child::_fkParentID ; ParentTable::__pkParentID]
Will create a new related record if no related record exists and will set the _fk field to the value that it already has if it does exist. Then your remaining script steps might be:
Go To Related Records [//specify child table and layout for child table options here]