if you are aiming to ‘show’ this hidden field when the user has selected ‘other’ from the drop down , goto the ‘drop down changes field data’ behaviour and then simply add in a simple branch..
add 2 ‘set state’ actions, and join these to the branch..
then on the TRUE line , set the condition for ‘if other is checked’ where the set state action is set to show the hidden text field, and then the FALSE simply needs to have the set state action to hide the text field again.
more details can be found in the behavior help file
and specific to the Simple Branch here:
You may also find that the training video’s here on behaviors will help as well: