It doesn't appear that deb_publications supports signing_service (https://docs.pulpproject.org/pulp_deb/restapi.html#tag/Publications:-Apt/operation/publications_deb_apt_create)
httpie example:
http $AUTH POST $BASE_ADDR/pulp/api/v3/publications/deb/apt/ simple=true signing_service=$BASE_ADDR/pulp/api/v3/signing-services// repository=/pulp/api/v3/repositories/deb/apt//
It would be great to have this available to us in ansible. if possible.