Issue with note placement when I am trying to write music

If you look at the attached image, the first line of music has two dotted half notes and an eight note in the same place. What I am trying to do is make the dotted half note be played at the same time as the eighth note, but for the duration of a dotted half note. However, when I attempt to place the notes where I want them, MuseScore does not allow me to do it. How would I do this in MuseScore?