Sending multiline POST requests by curl (in console):

curl -X POST 127.0.0.1:3000/api/v1/something/ -H "Content-Type: application/json; charset=utf-8" --data-binary @- << BODY
{
    "somefield1": "data1",
    "somefield2": "data2"
}
BODY

PS There was a nice GUI tool called Postman, unfortunately it became slow and bloated. Alternative: Insomnia. Version 2.5.0 is fine.