BM is a great tool for keeping track of weight loss. But weight loss doesn’t come on it’s own, it requires minding your calories and doing exercise. One of my favorite apps for the first, is www.lifesum.com: It helps me keep track of my calories intake just by entering what I eat every day. I’d be delighted if there would be a connector with this app (I don’t know if they have a public API, but they do connect to other apps, so it would be a matter of getting in touch with them).
It doesn’t look like Lifesum has an IFTTT or Zapier integration. Does it send confirmation emails or summary of your day’s activities? It might be possible to hack up a kind of unofficial integration, using IFTTT or Zapier, using something similar to what I described here:
It might not be exactly what you want (which may indeed require an official integration or a programming skills to get a script running somewhere) but you might be able to come up with something helpful. For example, if Lifesum sends you a daily email with a slightly different subject when you’re sticking with your plan vs. not sticking with your plan, you might be able to map those to a binary 0/1 Beeminder goal.