use the 'flesh' of your finger instead of the 'nail' or the stylus, when selecting the input
field

if the small keyboard is already up, press the middle button on the Dpad.

[provided you have 'launch via touch screen' & 'launch via rocker key' checked in the
thumb board settings]

I do not know if it is possible to make the thumb board the default even when either the finger's nail or the stylus is used.