How to add custom markers to the camera tracker? I need to have markers in specific places, and I can't add them.
Also if not possible, is there any other 3rd party tracking plugin which allows to add custom tracking points.
I assume your intended end result is not just adding markers, but attaching objects to spots that don't have markers generated by the 3D camera tracker.
If that is the case, you can create a 3D null from any point on a plane defined by at least three tracking points. The null will be aligned to that plane, and you can move it along that plane to the desired position using those two transform arrows that lie on that plane. Once in the desired position, you can attach a 3D layer to that null.
What I mean is to add custom marker before solving the camera, so I can have My own markers indicators to later add something on that specific spot. But Your explanation will also work, will need to change te position few times until it's on the right place, but it will work.