If you want the unit to have this stance from the beginning of the mission then it goes in the unit init field, otherwise you have to create a trigger with a certain condition.
The complete syntax is
this setUnitpos "MIDDLE"
Another suggestion: I understand that you are new to the editing, then I suggest you that, instead of asking "How do I do this or that?" every time you need to do something, you read first some editing guides (there is Mr. Murray editing guide which is very good) and learn something about scripting, otherwise you won't learn anything, and every time you need to do even the easiset thing, you will always have to ask here.
I don't mean to be rude, it's just a suggestion.