All right, once again IFTTT's maddeningly unhelpful error reporting has stymied me. Though the Maker channel was very easy to set up.
Indigo's restful API documentation shows "http://127.0.0.1:8176/devices/office-lamp?toggle=1&_
method=put" but IFTTT requires me to declare, via a drop down menu, if it's a put, a get, or something else. So I don't know if I should remove put, =put, method=put, _method=put, or &_method=put from the url.
IFTTT also wants me to declare what it is I'm sending from several options: application/json, application/x-www-form-urlencoded or text/plain. I guessed the latter.
Also, I discovered that almost all of my devices have spaces in their names. Insert %20 for each space?
Finally, no idea about digest authentication.
I'm way, way ahead of my knowledge here, but I see huge potential expansion if I can understand how to make this work. For one thing, though it's going the other direction out of Indigo rather than in to it, Logitech Hub is supported by IFTTT. Suddenly a whole new AV integration for Indigo!
Annnnd sending TO IFTTT works a trick: curl -X POST
https://maker.ifttt.com/trigger/TVon/with/key/[my secret API key] just turned on my AV system!