Insert Mode

On the Design tab, in the Edit Mode group, click Insert Mode to toggle Insert Points mode on and off.

Note that you can toggle snapping on and off using the I key. You can also Move String Points by simply"dragging" them to a new position in the display.

When Insert Mode is enabled and you move the mouse to insert a point on a string, the cursor will change to a crosshair cursor. Click on the string to insert a new point, then drag the point to a new location and insert more points as required.

Position the mouse cursor over the segment where you want to add a new point.

Click the mouse. You can move the inserted point by dragging the mouse before releasing the button. 

When a string is selected, you can also insert multiple points at a specified interval by choosing Insert Points from the right-click menu.