Ansible's known_hosts module's documentation is a bit scanty. Here's a more complete example on how to use the module:

- set_fact:
    host_IP: "{{ lookup('dig', 'host.example.org')}}"

# ecdsa seems to be the default if available on host.example.org
#
- name: get known_hosts fingerprint for host.example.org
  shell: cat /etc/ssh/ssh_host_ecdsa_key.pub | awk '{ print $1 " " $2 }'
  delegate_to: host.example.org
  register: host_key

- name: add host.example.org to known_hosts
  known_hosts:
    host: host.example.org
    key: "host.example.org,{{ host_IP }} {{ host_key.stdout }}"
  become: yes
  become_user: the_user_that_needs_to_execute_ssh