A colleague helped me with the knowledge that the customization 'existing_vm' key needed to be provide with a value of True, and the 'state' key needed a value of 'present'. The module below does set the 'administrator' password when the account is set for 'change password on first login':
- name: "Set a new administrator account password and power on the VM"
community.vmware.vmware_guest:
hostname: "{{ management_vcenter_ip }}"
username: "{{ management_vcenter_user }}"
password: "{{ management_vcenter_password }}"
datacenter: "{{ datacenter }}"
cluster: "{{ cluster }}"
name: "{{ vm_name }}"
state: present
networks:
- name: "{{ vm_network }}"
validate_certs: False
wait_for_customization: true
wait_for_ip_address: true
customization:
password: "{{ vm_password }}"
existing_vm: True
delegate_to: localhost