For UI (the delete button on portal row), I use a "+" button (icon really, not a button that does anything). Then I overlay it with the delete button which has a "hide object when"
For adding portal records, there is a method called Magic Key
The idea is to have a relationship (for just creating, but can be used for "editing".
and there are probably many ways to do as you ask.
as Beverly said - there are typically many different ways to accomplish "stuff" in FM. Here's a script that copies the primary key of the record you are on, then goes to the table underlying the portal, creates a new record, populates the foreign key, and goes back to the first layout and lands on the new row. The relationship to this child table has sorting turned off and "allow creation" is off. (The portal object needs to be given a name - in layout mode - in order to select it.)
That seems to be exactly what I am used to from VFP