It's common to procces form fields values in JavaScript before sending them to the server.
JavaScript natively makes use of JSon format to exchange structured datas.
Exemple: {key1:value1, key2:value2}.
The Content-Type header is then set to json: Content-Type: application/json

Exemple:

POST / HTTP/1.1
Host: localhost:8001
User-Agent: curl/7.58.0
Accept: */*
Content-Type: application/json
Content-Length: 34

{"key1":"value1", "key2":"value2"}

Curl:

$ curl --header "Content-Type: application/json" -X POST --data  '{"key1":"value1", "key2":"value2"}' http://10.10.1.11/