When I have already written notes and I want to add a time signature (4/4) with a pickup bar (one beat), I can do it with the panel on the right side unter time signature, but only when I click on the second note, but then the position of the time signature (4/4) is on the wrong position, it means at the beginning of the first complete bar and not where it should be, at the very beginning of the piece in the pickup bar.
Select the existing time signature itself before you replace it with the one you want to create with the additional pick-up bar.