I just found out the solution by my self: If you are using the type Service and just set the hostfilter, all services from the host(s) will be set in downtime.
url: "{{ icinga_host }}:{{ icinga_api_port }}{{ icinga_dt_url }}type=Service&filter=host.name==%22{{ item }}.{{ ansible_domain }}%22"
The curl command would look like this:
curl -k -s -u 'actionuser' -H 'Accept: application/json' -X POST 'https://SERVER:5665/v1/actions/schedule-downtime?type=Service&host.name='HOST'' -d '{ "author": "ME", "comment": "TEST", "notify": true, "pretty": true, "start_time": 1561012086, "end_time": 1561033686, "duration": 160, "fixed": false}'