Next left-click on the other edge. That will place a "Virtual Sharp" mark at the intersection of the two edges.
Now you can make your second selection for the dimension.
As I mentioned above, this method wasn't always available, so if you're using an older version of SW and "Find Intersection" isn't in the drop-down you can still get it done. Exit the Smart Dimension function, Ctrl+select both edges, and then choose the Point sketch tool. That will place a virtual sharp at the intersection of the two lines. Now you can dimension to it. As far as I know this has always been available.
By the way, you can select one of several virtual sharp styles at Tools > Options > Document Properties > Virtual Sharps.
The Virtual Sharp will come in as a pale gray color, but you can later move it to a layer with the desired properties if you want. Unfortunately as of SW2022 there isn't a way to assign a Layer to them in Document Properties.