How to make a line between object in Editor Window

If you’ve been using plugins like playmaker you know what I’m talking about

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But I can’t see how it’s done because the source code of playmaker is a DLL file

can someone point me in a right direction? thanks

Handles.BeginGUI( );
Handles.DrawLine(
new Vector3(startPos.x, startPos.y),
new Vector3(endPos.x endPos.y));
Handles.EndGUI();