With the Smart Dimension tool active, right-click on one of the edges and choose "Find Intersection" from the drop-down.
Then left click on the second edge.
That will insert a Virtual Sharp at the intersection of the two edges, and establish it as the dimension reference.
Now you can choose the second reference to place the dimension.
This workflow is a fairly recent enhancement (SW2015, maybe?). If you're using an older version then exit the Smart Dimension function, Ctrl+select the two edges, and then select the Point sketch tool. That will place a Virtual Sharp at the intersection of the edges, and now you can dimension to it.
By the way, you can choose which of several styles you prefer for Virtual Sharps at Tools > Options > Document Properties > Virtual Sharps.
Unfortunately, there isn't currently (as of SW2019) a way to set a default Layer or color for them in Document Properties. You can assign a Layer to them after they're placed if you don't like the default.