How to create a short bar

There are two ways that you could do this:

1) Enter a 2/4 time signature for the one bar. Then enter a 4/4 signature for the next. I'm guessing this is not what you want, but it's one way.
2) Right-click on the measure in question. (Not on a note or rest or text; right-click on blank space within the measure.) From the context menu, click on Measure Properties (in the UK version, it's apparently labeled "Bar Properties" instead?). Then change the "Actual duration" from 4/4 to 2/4 (or whatever other length you wanted). This is the same thing you do to add a pickup measure. The Handbook has info on this: https://musescore.org/en/handbook/4/pickup-and-non-metered-measures