Populate multiple items to a text field from a dropdown list

I am trying to create a dropdown list that when I select an item from the list it adds the item into a text box but doesn't delete the previous text box item.  I am just struggling with how to get the text field to concatenate and not simply replace the existing data with the new.  I thought it might be something I could do with action builder but it doesn't seem possible with out additional scripting.