The dropdown only returns an index or an UI string from the custom list. The actual magic has to happen inside your expression by e.g. associating the index with an array value or whatever. Expressions have no way to hard-set an external reference, so what you seem to have in mind won't work, anyway. You have to include all references in your expression and then pick the one you want to process via your code.
Thanks Mylenium. I think I used the wrong syntax for my description. What I'm actually looking for is a way to fill the descriptive names of the items - I know, that the "values" are actually the indeces, thats fine.