It used to be https://www.twitch.tv/babymetalband, but now it’s https://www.twitch.tv/bifuteki3.
They only used it once in early April 2016 for this Q&A stream:
KOBAMETAL made an appearance twice and answered some questions.
Gimme Chocolate!! ("10 BABYMETAL BUDOKAN" ver.) is out today on digital and streaming services.