Here is a video illustrating this process. Written instructions are below...
If you would like to add a measure, bar line, new page, or system in Notion or Progression for desktop, here is what you will want to do:
1. In the palette at the bottom of the screen, you can click on the pane that has all of the bar lines and repeat bars in it to select the single bar line. You will notice that you can also press the letter "i" on your computer keyboard to arm your mouse pointer with a single bar line. You can repeatedly press the letter "i" on your keyboard to toggle between the different bar lines.
2. Once you have pressed the bar line key or the letter "i", you can then simply move your mouse into the staff where you would like to add the bar and left-click. You can add as many bars as you would like. Adding bar lines will automatically give you a new system or a new page.