How to add a half note under an eight note on the same beat?

I just started playing piano a month or two ago after having had harmonium lessons for about two years over 40 years ago. In other words, my music skills and knowledge are limited and I have never used MuseScore software until today.

I think my problem is best explained with a few screenshots:

And then the question is: How do I add that half note without it removing all the other notes?

I've tried a few things but it either changes the other notes into something else, pushes them forward/backward or just removes them like in the example screenshot.

Thank you for any help.