How can I create / update a document with a Dropdown list that has "Multi Select" enabled?

I have a UI of a Travel and Itinerary Planner for my university requirements which has the UI of like this:

So in the button when submitting an action, I have a problem, kind of, I can't update nor create a document with having the values of the Dropdown list that has "Multi Select" enabled because it shows up like this in the actions:

It has a red fill on the variable and it has an empty field when clicked.

How do I create/update a document that has "multiple" selections on the dropdown?