Hi Magesh,
As Shravan suggested, currently supported fields are limited in the api for event registration:
POST ${endpoint}/EvtMgmt/api/v2.0/events/${eventID}/registrations
You can find that acceptable fields for contact entity are restrict to firstName(instead of firstname), lastName and email, which means Event API is standalone compared with general Web API.

Actually from my test,
in test 1, I have
- added a custom field Job Title
- added jobtitle attribute to Attendee.ts(an existing OOB field of Contact)
- added jobtitle in formdata in attendee.component.ts
I could still create a new event registration record successfully, and jobtitle has been added in request JSON body,

but only 3 default fields have been accepted when I check the record in CRM.
In test 2, I tried to send direct request to add contact with jobtitle in console,
however, the field has still be ignored even thought I got success callback.
var attender = {
attendees: [
{
firstName: "123clofly",
lastName: "mao",
jobTitle: "Manager!",
email: "123456@test.com",
passId: "",
waitlisted: false,
autoRegister: false,
responses: []
}
]
};
var serverUrl =
"https://endpoint/EvtMgmt/api/v2.0";
var token =
"Token";
var odataSelect =
serverUrl "/events/Ted_Contoso/registrations?emApplicationtoken=" token;
$.ajax({
type: "POST",
contentType: "application/json; charset=utf-8",
data: JSON.stringify(attender),
datatype: "json",
url: odataSelect,
beforeSend: function(XMLHttpRequest) {
XMLHttpRequest.setRequestHeader("Accept", "application/json");
},
success: function(data) {
console.log(data);
},
error: function(errorThrown) {
alert("Err: " errorThrown);
}
});
So currently I suggest you could use landing page as alternative to default registration form,
then trigger a workflow to create an event registration record from new contact.(If contact's Originating event field contains data)
Regards,
Clofly