Webform Submission Using Rest API POST method
Looking at the examples on the Webform REST project page, you should be using something like…

{
  "webform_id": "my_webform",
  "checkboxes_field": [
    "Option 3",
    "Option 5"
   ],
   "integer_field": 3,
   "radio_field": "Mail",
   "email": "myemail@mydomain.com.au"
}