Nevermind, I figured it out. It's a bit hard to find but very easy to do. Here's the steps for anyone who needs help with this too.
1. Click colermein on the drawing
2. Don't do anything, just click File>Save Oekaki>Stop drawing, I'll finish it later
3. Go to the forum you'd like and click NewDrawing
4. You should be able to choose the base from unfinished works
