Mailchimp App - interests/groups - how to link to checkboxes

Hello,

I'm trying to subscribe a user to Mailchimp list and let them choose which groups/interests they want to be added to.

The Perch docs for this suggest using a hidden field:

<perch:input type="hidden" id="cheese" value="a375f7a461" mailer="interests" />

but I'm assuming that's almost an "auto-submit" method, where you'd hard-code the interests you want the user to be added to.

I've tried to change the above to a checkbox:

<perch:input type="checkbox" id="cheese" value="a375f7a461" mailer="interests" />

but this is causing an invalid response from the Mailchimp API:

[body] => {"type":"https://developer.mailchimp.com/documentation/mailchimp/guides/error-glossary/","title":"Invalid Resource","status":400,"detail":"Invalid interest ID: '_empty_'.","instance":""}

so my ID isn't getting through.

Would I need to wrap the original interest hidden field in a Perch if and then only output it if a checkbox is checked?

If that's the case, I'm not sure of what syntax to use because Perch checkboxes and ifs look to be somewhat different if they're inside a Perch form. I've tried this, with the same _empty_ result:

<perch:input type="checkbox" value="1" id="choose_cheese" />
<perch:if id="choose_cheese" value="1">
    <perch:input type="hidden" value="a375f7a461" mailer="interests" id="cheese" />
</perch:if>

Thanks for any suggestions,

Gareth