curl --request POST \
  --url https://api.virtuoussoftware.com/api/VolunteerOpportunity/{volunteerOpportunityId}/Volunteers \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "contactIndividualId": "<integer>"
}
'

{
  "id": "<integer>",
  "volunteerUrl": "<string>",
  "createDateTimeUtc": "<dateTime>",
  "createdByUser": "<string>",
  "modifiedDateTimeUtc": "<dateTime>",
  "modifiedByUser": "<string>",
  "contactId": "<integer>",
  "contactUrl": "<string>",
  "displayName": "<string>",
  "firstName": "<string>",
  "avatarUrl": "<string>",
  "primaryPhone": "<string>",
  "primaryEmail": "<string>",
  "totalHours": "<double>",
  "timesVolunteered": "<integer>",
  "volunteerAttendanceUrl": "<string>",
  "volunteerOpportunityId": "<integer>",
  "volunteerOpportunityUrl": "<string>"
}