Add Text with Command button

I have a Form, I'll call it "4". Within this Form is a sub-Form, I'll call
it 
"4a". I have a couple different buttons on Form "4" which effect Form "4a"
in 
different ways.

One of the fields on Form "4a" has the name "comments". I want to add a 
Command Button to Form "4" that when clicked puts text in the "comments" 
field. Since "comments" is a text field Users will be able to enter
whatever 
they like but if they just want to enter the default text they can just
click 
the Command Button and the text will be entered.

I'm certain this is easy but for some reason I can't think of it.

Thank you!