How to add this note to musescore?

There are three notes and their names depend on the clef used. A correct answer is impossible.
So provide more details so that we can help you.

To enter chord notes press the name of the lowest and then together with the shift key the names of those above or place the notes by using the mouse.

If you are trying to have the cord and rest on the same staff, you can't do that in Musecore as far as I know. If there are multiple different notes or rests that start at the same time, you need to add a different staff. Else, if you are just trying to create the cord, minus the rest, just select the whole note on the top bar and then click the places where the notes go on the staff.

I just found out that you can put the cord and rest on the same staff, for the cord, just place it, and up on the top bar on the left, where there is the quarter note with a one and a quarter note with a two, press the quarter note with the two. These in Muscore, are known as voices, it allows you to use the same staff for multiple of the same instrument, or just the other hand, your cord is in voice one and your rest is in voice two.