Hi, I'm a newbie finding my way round, graduating from W10 Paint with some difficulty. Alphas and plug-ins are way above my pay-grade at mo. What you could try is magic wand a white portion, either contiguous or global as appropriate. Then simply delete the magic wand selection, which then becomes transparent. Also try try tinkering with tolerance for the magic wand selection as there's no gradient between solid off-white and transparent no pixel.
My problem why I'm here is dear old Paint's transparent/opaque toggle, which I can't find here. Even copy/paste or move a selection on a transparent background obliterates everything in the selection area. You paste transparency, not nothing, if you see what I mean. Paste in new layer works a bit clunky, but even more fiddly to "toggle" back to opaque.