Every time we reply to your ticket you get an email. You can reply directly to that email and your message is added to the same ticket — you do not need to sign in.
Keep the subject line as it is. It contains your ticket reference, which is how your reply finds its way back to the right conversation.
Attachments work by email too. If you are sending a screenshot, attaching it to the reply is fine.
This is general guidance, not a guaranteed fix. The same symptom can have different causes on different machines. If you are unsure about a step, or it does not help, raise a ticket and tell us what you tried.