The simple way to use this is click the (+) on the right. This will open a list showing variables for different modules and plugins.

Hover over TP Vtube Studio Plug and a new list should show up.

Hover over Expressions and click on the expression you are wanting to get the state of.

This will add a string similar to

When activated this will set Expression State variable under Current Info to the state of the expression (True|False)