Score Markings

Hi there. I am trying to set the attached excerpt from a Brahms string quartet in MuseScore. There are two markings that I am not sure how to add. The first is the slash mark above the whole notes in bars one and two indicating that they are subdivided. The second is the quarter note at the root of the G-En-C chord in measure one. Every time I try to add this note it screws up the half notes on top of it.

Any help would be appreciated!