Turn on signposts if not on. I’m assuming 4/4 time signature. During note entry just after entering the 3rd beat of the first ending (the caret will be on beat 4), press shift-b ‘:|’ to put in a repeat. That will create a 4/4 measure with one beat after the repeat. Delete the time signature signpost. Click in the incomplete measure (selecting the entire measure) then press Shift-R ‘ending’ to put in the 1st and 2nd endings.
Go to Library > Notation Options > Repeats and set Clefs and key signatures in repeat structures to Do not track. Given that a time signature is neither a clef nor a key signature, I don’t know why this change in setting should be necessary.
