That would be a Custom Skill, correct? So, you need an invocation word as in “Hey Google | Alexa, tell ISY to do XXX_ARBITRARY_UTTERANCE_XXX”, right?
We ditched an early Custom Skill and moved to the Smart Home Skill because of the improved cognitive ease without the invocation word. (Plus, people expect to see the device appear in the Google Home / Alexa apps and control from that UI alongside other devices.)