I suppose, you should use a different URL path for posting issues, please see here for post method. It should be like this: https://api.bitbucket.org/2.0/repositories/username/reponame/issues
For posting just an issue with a title you could use this:
curl --ssl-no-revoke -u username@password -X POST -H "Content-Type: application/json" -d "{\"title\" : \"test2\"}" https://api.bitbucket.org/2.0/repositories/username/reponame/issues
To post a content of the issue use content tag:
curl --ssl-no-revoke -u username@password -X POST -H "Content-Type: application/json" -d "{\"title\" : \"test2\",\"content\": {\"raw\": \"just test text\", \"markup\": \"plaintext\"}}" https://api.bitbucket.org/2.0/repositories/username/reponame/issues
For example for me this worked (it's my private repo):
curl --ssl-no-revoke -u myaccountname@mypasswordname -X POST -H "Content-Type: application/json" -d "{\"title\" : \"test2\",\"content\": {\"raw\": \"just test texts\", \"markup\": \"plaintext\"}}" https://api.bitbucket.org/2.0/repositories/dvmochalov/testrepo/issues
--ssl-no-revoke - works only for windows curl and it's just in case if you are running Windows with antivirus software or working with proxy.