cmb2_init_hookup_{$this->cmb_id}

See hook in action

See this hook used in plugins:

Select Plugincmb2.2.4.2cmb2.2.5.0cmb2.2.5.1cmb2.2.6.0cmb2.2.7.0cmb2.2.8.0cmb2.2.9.0cmb2.2.10.1