Hi FlowerPi,
Here is a sample request to add attendee.
Prerequisites:
Disable CAPTCHA.
URL:
https://abcdefg.svc.dynamics.com/EvtMgmt/api/v2.0/events/{readableID}/registrations?emApplicationtoken=xxxx
Please read this article for how to generate an Event API endpoint and get emApplicationtoken.( It authenticates your web application against the event management public API.)
https://docs.microsoft.com/en-us/dynamics365/marketing/developer/register-web-application-events-api
Method:POST
Request payload:
{
"attendees":[
{
"firstName":"Cloyer",
"lastName":"Mao",
"email":"001web@test.org",
"passId":"",
"waitlisted":false,
"autoRegister":false,
"responses":[
],
"attendeeSessions":[
]
}
]
}
Result(JSON callback data)
{
"errorCode":null,
"status":"Success",
"errorMessage":null,
"Message":null,
"redirectUrl":null,
"purchaseId":null
}
Status code for success is 200.
You can also refer to page 10 of PDF documentation.

Regards,
Clofly