New Post: Use DataIdMultiSelector as codebehind parameter?

My goal with my function is that when you insert it in the editor you get to set the order of the items in the DataIdMultiSelector in the popup window. What do you have to type in my codebehind to achive this?

The datasource for the DataIdMultiSelector is always "C1DB.Posts.Categories" so I only want the user to be able to select what categories to be added to the function (UI mode = compact).