The oval button on the symbols palette inserts an invisible root: a bass note printed with no chord symbol above it. Use it for a moving bass line under an unchanging harmony, so a passage reads C, then /B, then /A.

 

Press w and then the bass note, so w/A gives you /A. You can also click the oval on the palette, type the note, and press Return.

 

For more, see Chord symbols and notation on the iReal Pro website.