This button pops open a Brandbot window. Brandbot offers to log you in with Mindbody credentials, or set up a new account, which feeds into MindBody.
The popup, though, seems to be designed by the Brandbot plugin, not the MindBody interface.
This payment popup, though, works great on Mobile.
Is Brandbot using some form of MB data to make the field? Or is the BB data being sent to MB?