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You can't. Draw on it in 3D. Or, draw it in a different family set to workplane based, then host it on the sloped (and named) workplane in your main file.
You can kind of do it with the workplane viewer, but are pretty limited in what you can actual do on it. Pretty much sketching only.
Name the RP. Then open a 3D view, right-click on the View Cube, choose Orient to a Plane from the context menu that appears, and the select the RP's name.
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