def METHOD_NAME(register):
    field = AppointmentsConfig._meta.get_field("plugin")
    assert isinstance(field, AppointmentBackendChoiceField)
    validators = [
        validator
        for validator in field.validators
        if isinstance(validator, PluginExistsValidator)
    ]
    assert len(validators) == 1, "Expected only one PluginValidator"
    validator = validators[0]
    with (
        patch("openforms.appointments.admin.register", register),
        patch("openforms.appointments.utils.register", register),
        patch.object(field, "registry", register),
        patch.object(validator, "registry", register),
    ):
        yield