Ansible: if env var is not set prompt for value

ifunset.yml

---
- hosts: localhost
  tasks:
    - pause:
        prompt: "What is the GitLab registration token? "
      when: ( lookup('env','GITLAB_REGISTRATION_TOKEN') == "" )
      register: reg_token
      no_log: true
    - set_fact:
        gitlab_registration_token: "{{ reg_token['user_input'] }}"
      when: ( lookup('env','GITLAB_REGISTRATION_TOKEN') == "" )
      
    - set_fact:
        gitlab_registration_token: "{{ lookup('env','GITLAB_REGISTRATION_TOKEN') }}"
      when: ( lookup('env','GITLAB_REGISTRATION_TOKEN') != "" )
    
    - debug:
        msg: "System {{ inventory_hostname }} has reg token {{ gitlab_registration_token }}"