Can't selecting Multiple Dropdown with Selenium webdriver (not class Select)

I'm having a problem with my tests with Selenium webdriver. I'm using Java. Can't select from a multiple drop-down that is not class Select. This is how the drop-down looks like:

And that's the code:

This is how it looks like when 2 options are selected. I was wondering if I can try with KEYS but the page doesn't work like that. Haven't seen that kind of field before, and not sure how to proceed?