I'll have a crack at this but be on the look out for better advise from more experienced respondents.
If you have two tables
with field pkID
with fields pfID and fkIDParent
You could relate them to each other like this Parent::pkID = Child::fkIDParent.
Now you can make a layout of Parent table and place a portal of Child records in it. If in the relationship specification(edit) dialogue you select "allow creation of records in this table from within the other...". You can create/edit child records within the portal. Filemaker seems to prepare the next portal row for immediate data entry so this may suit your needs.
Another option is to explore script triggers and or buttons and create a script that creates a new record in your table. You can then assign a trigger (or button) to launch the script and create your new record on something like "on object exit" or "on record commit"
Also check out 'table view' for your layout ('View As' in the dark grey bar second to top of window).
In this view your next record is prepped ready for input but not created until you actually begin inputting.