Welcome to the site. IIRC you have already asked about that sending a ticket. Have you read that?
Ok... once again ... if you are in a thread there is a separate window for posting at the bottom of the page there. There are two buttons displayed "Attach files" on left and "Post replay" on right. Please type your text in the field above the two buttons and while all is written hit the Post replay button. If you want to replay to a particular post there you have to use the "Reply" option displayed at the right bottom of each post. The particular post is quoted in the answer window so all you need to do is to write your message below the quotation and hit the Post replay button at the bottom.