There are two ways to create a comment in OTRS from within JIRA:

Via the "OTRS Comment" Action

1. An OTRS comment can be created by selecting OTRS Comment in the "More" menu.
    
   The "Add a comment to the OTRS Ticket" dialog is displayed.
   
2. In the "Add a comment to the OTRS Ticket" dialog, you enter the subject and the comment itself.
   As long as there is no subject the comment can not be submitted.
3. Select the "External note" checkbox to mark the comment as external note (which customers can see) in OTRS. The comment also appears at the JIRA issue.

Via JIRA Standard Comment

Standard JIRA commends will be transmitted to an OTRS ticket as long as JIRA Comment Event is selected under "Comments Events" in the JIRA add-on configuration.