crm_core-8.x-3.x-dev/modules/crm_core_activity/crm_core_activity.links.action.yml

crm_core_activity.add:
  route_name: entity.crm_core_activity.add_page
  title: 'Add an activity'
  appears_on:
    - entity.crm_core_activity.collection

entity.crm_core_activity_type.add_form:
  route_name: entity.crm_core_activity_type.add_form
  title: 'Add activity type'
  appears_on:
    - entity.crm_core_activity_type.collection