Our API works great with curl. Use -i and -H to specify your key and user in the header, then use -X to specify the API action. If you're posting a payload use -d and then the file name. The second example uses a file in the same directory the curl command is being run from.
curl -i -H"Api-Key:YOUR KEY" -H"Api-User:YOUR USER" -X GET https://api.yesmail.com/v2/subscribers
curl -i -d @YOURJSON -H"Api-Key:YOUR KEY" -H"Api-User:YOUR USER" -H "Content-Type: application/json" -X POST https://api.yesmail.com/v2/emails/send