To add a user defined Guide Line Segment use Shift + ` (the button next to 1, above Tab)
To force a Guide Line mid command (i.e. when placing a wall) use just “`”. Start to place your wall, hover over a point (end or intersection etc), hit the single shortcut key, and the Guide Lines will appear as you move your mouse, to make your desired Guide Line stay, simply click on it. Now it will change from blue to orange and it will remain until you have completed your command.