How to create number with strike-through line for harmony change?

Assuming you are using the Figured Bass facility (Ctrl+G), then you can do that by simply typing a s;ash after the number. But see the Handbook under "Figured bass" - apparently slashes through the digits 1-4 aren't supported, or are converted to + instead. I'm not familiar enough with figured bass to say if there is a good reason for that restriction.

@happysherry921, I do not know if that's what you are looking for. I tried it with Ctrl + L and entered \ (changed size).
Perhaps best press Z and use a symbol:

There are a lot of variations in how to write figured bass notation, and no two continuo players will agree on everything. Still, professionals will recognise most of the different ways of augmenting or diminishing an interval number (/, \, +, #, b) from context, so the music will get played properly even if the harpsichord part gets marked up in the performer's preferred style during the first read-through. ;o)

MuseScore does not currently support combining slashes for the figures 1, 2, 3, or 4; these figures can only be altered with a + sign. Backslashes (\) can be combined with 5, 6, 7, 8, and 9; a forward slash (/) can ony be combined with the figure 5.

For complete information on how to use the figured bass feature in MuseScore, see https://musescore.org/en/handbook/figured-bass