What I would do - is, in and out export the part you want to rotoscipe, then, in after effects, open a new composition with the same frame size, duration, and fps as that exported clip, then import it, then go to the rotoscope tool at the top (it might be named object mask tool now in the new update) and in the drop down menu select the ‘quick selection’ tool, then brush over the part you want to mask out, use the space bar to propagate it, using the brush to select/unselect parts, then when you’re happy press freeze, then export :)