curl --verbose -X POST --data "field1=value1&field2=value2" http://www.example.com/form.html
For APIs that expect JSON bodies:
curl \
--data '{"field1":"value1","field2":"value2"}' \
--header "Content-Type: application/json" \
--verbose \
-X POST \
https://www.example.com/api/endpoint
Use --data @filename
to send from a file.
Use --data @-
to send from stdin.