It depends what OS you're using (iOs or Android)
I think this trick only works with Android devices.
The way to do this is to copy and paste/insert an entire line at the beginning of the 12th line. It will then move the 12th line to next line (13th line). Keep in mind that you cannot edit the 13th line once it's been inserted. So you need to make sure your line is correctly edited before inserting it (otherwise you'll have to undo and redo the process again)