If you want to add new row to child. THEN you want to open that form, then you can consider different approach. My suggetion was just to open new form to the child by action. Unless the user save the form (if user can cancel the form) then nothing happens.
If you like to add the new child row first, then want to open that newly added row, think about this workaround.
1st action – Add new row to the child table. For the ref field in child, pass the parent ID value. At the same time, for the child record ID, pass this value (expression) of
Then create another action with deeplink.
Those two actions are hidden.
Then last action to GROUP those two action and place it wherever you want.
By firing action the new child record is added and saved, and you are prompted to the form view to edit. Even you cancel the edit, the row is down there, as it is already saved before we go to the form view.