Using the Angle Bisector relative construction method, you can draw elements bisecting the angle formed by other elements or temporary reference vectors.
Note: The Control Box is not visible by default. It is available at Window > Palettes.
You can either start drawing the new element before activating the Angle Bisector method and defining the reference vectors, or the other way round. Before using Angle Bisector constraint, make sure that Grid Snap is disabled.
Click the Angle Bisector icon, then define the reference vectors by either clicking on two existing element edges or lines, or drawing two temporary lines. While you are clicking or drawing the reference lines, the cursor icon will show that you have activated the angle bisector constraint.