Finally I ended using (it's also working for different languages of Recaptcha challenges) if someone searches for the same:
">CSS>iframe[src^=' https://www.google.com/recaptcha/api2/bframe ']>FRAME> >CSS>#recaptcha-verify-button"
@stephenperry18 You need to attach to something else, which is not changing.
For example, if there is only one frame on page, or index of frame is not changing you can use following expression:
