Fly a straight line for a specific distance - e.g., 300 feet

I want to fly a rectangle of exact measurements. I can use the shift-click option to constrain the waypoints to be at exact right angles. Is there a way to constrain a straight line path to a specific length?

You can’t in the Mission Hub.
You could use the Site “Calculating with coordinates”
to calculate the coördinates of a projected point (Calculation Option 3. Projecting a point)