How can I add a dedicated Slack channel to a Jira Issue using the Jira REST Api?

[s_dantchev]

How can I add a dedicated Slack channel to a Jira Issue using the Jira REST Api?
I try the requests from the web page but I am not successful.

E.g. https://jira.dkcorpit.com/rest/slack/latest/oauth/begin/XXXX
returns 500
It obviously requires oAuth but I fail on the first step. Probably there is a separate way for the REST Api.
I am using a bearer token authentication.
I try POST https://myjira.xx/slack/dedicatedchannel/
with the proper payload but returns 

{

    "statusCode": 400,

    "message": "Could not load slack link or user. Account is not linked"

}