If you are a student, you can access Blackboard Collaborate Ultra through your school's LMS (learning management system). Blackboard Collaborate Ultra is a plugin for the Blackboard Learn LMS, so if your school uses Blackboard Learn, you can access Blackboard Collaborate Ultra through that.
If you are a teacher, you can access Blackboard Collaborate Ultra through your school's LMS as well. If your school does not use Blackboard Learn, you can still access Blackboard Collaborate Ultra by going to the Blackboard website and signing up for an account.