How to set up a callin.io scenario to send custom voicemail messages with callin.io calls?

I'm initiating callin.io requests via Zapier. When a lead comes in, I use Zapier to start a call to that lead through callin.io. My goal is for the AI to leave a pre-recorded voicemail message if the call is answered by a voicemail system rather than a person. Within callin.io's call settings, there's a field for a custom voicemail message. However, when I send a call using Zapier, although I can enable machine detection, I don't see a way to input my custom voicemail message. Could you please assist me with this? How can I set up the system to ensure the custom voicemail message is played when the call goes to voicemail via Zapier?