As default every measures starts with a rest ( a whole as default if you have a 4/4 time signature )
So in a new measure, you have to change the duration of that rest..
so just click on the existent rest, and click the "quarter" toolbar button. ( You can also configure keyboard shortcuts for all durations, and "Increment Duration" "Decrement Duration" + and - as default )
Note that there is a known bug:
if the duration button is already selected, by clicking it nothing happens. so in that case you could select 1/8 and then 1/4 to temporally fix the problem.