In Blender 2.79 used to be an At field for placing custom shaped bones at different bones. In the manual of 2.83 there is still a reference to it - see below copied text of the manual
Workflow To assign a custom shape to a bone, you have to: Switch to Pose Mode Ctrl-Tab. Select the relevant bone by clicking on it. Go to the Display panel Custom Shape field and select the 3D object previously created in the scene; in this example we are using a cube and a cone. You can optionally set the At field to another bone.
However, in my version (2.83.1) there is none. Was it moved to a different place or is there a different way to place a custom shaped bone at a different bone?