To add a pickup measure into a newly being created song, perform the following steps on New Song Screen:

1. Turn on the "Pickup measure" toggle.

2. Set time signature to the pickup measure.

3. Press "Done" to navigate to the song composition screen. When you add notes into the song, they will be automatically calculated so that the first measure has the time signature of the pickup measure you have set.

Demo video:

Thanks for using the app!