Giving Dropdown Widget string selection as an input to a Service

Dear developers, 

I wanted to substitute a TextBox with a Dropdown widget, as the String can be only between two values. However, I am struggling in finding a way to provide the current selection as an input parameter to the Service, as for example the SelectedText field can be only have a Source but not a Target binding. I have found some solutions around but I quite do not understand them.

Thanks and BR