Treble clef

You need to add a separate stave, then declare it as a Cutaway (in Stave/Part Properties > Stave Properties > Cutaway). See the Handbook on this topic:
https://musescore.org/en/handbook/3/staff-part-properties#common-staff-…

It's not really clear without more context what this is trying to indicate, but it seems those measures are meant to be played before, not simultaneously with, the staves below. So I would add a couple of measures before the current start, and a horizontal frame to keep them from extending to the right edge of the page, then use the hide empty staves feature to make the other staves not show. Exactly how to do this depends on the rest of the context - where there are other empty measures you don't want to hide, whether other instruments appear later, etc.

In some cases, it could be simpler to just notate that elsewhere and snap a picture of it using the image capture tool and then paste it in where desired.

Use [Edit]>[Instruments] to add a new staff and move it above the grand staff. Add any notation in the new staff and then adjust its "hiding" properties in the staff properties - see https://musescore.org/en/handbook/3/staff-part-properties#common-staff-…. You will probably want "hide when empty = auto". While you are editing the staff properties you can also assign the appropriate instrument. Then in [Format]>[Style]>[Score] tick the "Hide empty staves within system" option.

However, is that small staff really supposed to be played at the same time as the grand staff. It says "Trumpets before each stanza." Doesn't that mean it is played before whatever is in the grand staff?