To add CAPTCHA to forms using the Webform module, for example contact forms:

    • Install and enable the Captcha module
    • Go to settings of the CAPTCHA module /admin/config/people/captcha
    • To ensure, CAPTCHA is used in every possible and available way,
      • Enable:
        Default challenge on non-listed forms.
        Enable CAPTCHA for every form not listed here
      • Save
    • In front-end view, navigate to your webform where CAPTCHA is missing. We need to identify its Form ID to be used in the next steps
    • To get the Form ID
      • View source, and find or search in the code: form_id
      • After the code name=”form_id” there should be a value shown as: value=”some_weform_value_here” (see example screenshot)
      • Copy that value to be entered in next step
        Captcha form id for Webform in Drupal 8
  • Go to Form Settings /admin/config/people/captcha/captcha-points, where all forms having the captcha enabled or made available by default
    • Identify the contact webform, e.g. contact_message_feedback_form
    • Edit, and paste the copied value from previous step into the required Form ID
    • Save
  • Clear cache, and check that default CAPTCHA is now available and displayed

This is a simple explanation, for clarity, of the instructions as posted here: https://www.drupal.org/project/captcha/issues/2736881#comment-13594561