UPDATE 2
SOLVED, I was missing Header String tutorial instructions at https://developer.twitter.com/en/docs/basics/authentication/guides/authorizing-a-request.html
Answer to my question: Headers must not contain any json data.
Here's my fully functional project, just insert your credentials and you are ready to send Twitter direct messages: https://github.com/ricarrrdo/gs-rest-service
Note: I'm using Spring deprecated AsyncRestTemplate, so if someone knows how to use the new WebClient, please upgrade restASyncClientBody() method inside Application.java - it would be much appreciated!
UPDATE 1
https://github.com/ricarrrdo/gs-rest-service/blob/master/src/main/java/hello/Application.java#L2-L12
This link above is my spring boot project which recreates Twitter given example at https://developer.twitter.com/en/docs/basics/authentication/guides/creating-a-signature.html to authenticate and authorize a API request.
It's working fine, just compile/run with mvn spring-boot:run and watch console output.
You can check lines 41-52, which are the same parameters used in Twitter example.
Problem/Question
I'm trying to send a Direct Message on Twitter via Twitter API but I keep getting Bad Authentication data error. How should I change my code to be able to send direct messages?
I mean: In https://developer.twitter.com/en/docs/basics/authentication/guides/creating-a-signature.html there's an example with how to create a valid Oauth header using x-www-form-urlencoded parameters. But how can I create valid Oauth headers when sending Json?
Twitter tutorial for direct messages here - https://developer.twitter.com/en/docs/direct-messages/sending-and-receiving/api-reference/new-event
Notes
Authorization header needed to send a direct message:
authorization: OAuth oauth_consumer_key="YOUR_CONSUMER_KEY", oauth_nonce="AUTO_GENERATED_NONCE", oauth_signature="AUTO_GENERATED_SIGNATURE", oauth_signature_method="HMAC-SHA1", oauth_timestamp="AUTO_GENERATED_TIMESTAMP", oauth_token="USERS_ACCESS_TOKEN", oauth_version="1.0"
