Addition of chord in between key and time signatures

You'd need to create and use an im age for this, MuseScore doesn't support this directly

It is possible with a workaround though an image might be easier.

Start in a measure with a 2/4 time signature.

Use an unused voice like voice 2 (See https://musescore.org/en/handbook/3/voices) to create a 1/4 note chord. Select one note in the chord and in the inspector (F8) adjust the checks so the chord is set to Stemless and small. In the note section set it to not play and then for each note in the chord change the Head type so the bottom one is quarter and the rest are whole.

You can now make the extra rests in voice invisible by selecting them and pressing v

Now in the chord section, adjust the x offset of the chord so that the notes will be where you want them and there is room to add parentheses. The number will be something like -6 (that's only a guess).

You can then click the time signature. If you are having trouble clicking the time signature because MuseScore insists on selecting a note, press ctrl while you click in the same spot that selected a note and the selection will change to the time signature. Once that is selected you can change the x offset of the time signature to something like 6.

The final step is the parentheses. There are a few ways to do this, but I suggest that you use staff text (select a note in the chord and press ctrl+t) type ( then a space then ). Press esc to exit text entry mode. Make sure the parentheses are still selected and in the inspector change the font size to a number like 36. Once again, this is a guess, you will probably need a bigger number. If you don't have enough space between the parentheses you can put the text in edit mode while it's selected. The handbook says pressing ctrl+e does this, but I think that shortcut was changed to something like ctrl+alt+e or alt+shift+e in version 3. I think I changed my shortcuts so ctrl+e works. Once the parentheses are in edit mode you can add spaces if you need to so they will be wide enough to fit around the chord. Don't forget, you can still adjust the x offsets of the chord or time signature if you need to.

Finally drag the parentheses down so they are around the chord.

You now have the first chord set up. It gets easier at this point.

Open the filter selector (F6) and remove the check only from voice 1. Select the entire measure you just set up (click a note in the chord and shift click the last rest works but there are other ways). Copy the selection (ctrl+c).

At this point I suggest that you move the time signature in the measure you will paste to the same amount as you did in the first measure. You can then click a note on the first beat of the destination and paste (press ctrl+v) and everything should be set up good. One thing to look for is that if the measure is longer than 2/4, you will have a visible rest in voice 2. click the rest and press v to make it invisible. You can select each note in the chord and change its pitch with either up and down arrows or ctrl+up and down arrows as appropriate. Once again, you can change the x offsets of the chord and/or time signature as needed.

Continue pasting and adjusting until all of the staves are set up, then remember to use F6 and check All so future selections will work properly.

It sounds quite complicated, but once you do it a couple of time you'll understand everything. Feel free to ask questions if something doesn't work right or you don't understand something.