This is easy to set up if you are using a portal for this. Please look up portal in FileMaker help to find more background info on this very useful layout object.
If you have this relationship: Parent::PrimaryKey = Child::ForeignKey
In Manage | Database | Relationships, you can double click the line linking these two table occurrence "boxes" and enable "Allow creation of records via this relationship." for the Child table. Then you can simply start entering data into the bottom blank row of a portal to Child placed on the Parent layout, and a new record in child will be created with the value of PrimaryKey will automatically be entered in to ForeignKey.