Javascript for dropdown boxes

I'm new to Acrobat Pro XI,

I've created a form and I would like to transfer the information selected from a dropdown box , once selected, into a text box using Java script. If possible can you have 2 selections from a dropdown entered into the same text box?

I've tried various different Java scripts but they don't work. What would the correct Java be? Where should I enter it using Acrobat pro XI?

Many Thanks

Mark