trill with parenthetical note

I have never done this until now. What have you tried?

A parenthetic notehead appears automatically if you select an interval greater than a second. In your case it is a minor second and that would normally be notated with a flat above the trill sign.

To get what is in your picture you would need to
- add the desired note in voice 2,
- in the properties panel make it stemless,
- add parentheses in the notehead section,
- untick play,
- tick cue sized,
- adjust the horizontal offset of the note and the accidental (they move independently),
- add the trill line as normal and select minor second as the interval.
- This will add a flat sign above the trill sign. You will need to make that invisible.

After all that you should end up with this.

I have also attached the score where I did that so that you can see what is what in the properties panel.
FlatTrill.mscz

It is much easier to just have the default accidental on the trill of course.