Basic uneducated newbie needs education. :-(

Hi all, completely new to this so i apologize for being completely ignorant.

I am taking guitar lessons and was given a hand written arrgmnt of "Star of the County Down". It's hard to read/understand so i thought if i could get it on sheet music it might help to make sense.

what i am trying to figure out, without knowing what any of it is called, is this:

Lets say i have a 2 beat measure.
I have a half note at beat 1 and want to add a quarter note at beat 2. Muse score thinks this is an attempt to put 3 beats in a 2 beat measure.

The way the score is written on paper is the first note is a half note at beat 1. It has a quarter rest underneath it. the quater note at beat 2 is associated with the Quarter rest and is played at beat 2, while still holding down the first note.

How do i tell Musescore that this is not breaking the laws of music?

Hope that makes sense to someone?

Thank you so much for your patience, understanding, etc.
Poppa