Opening a ticket
Start one from the help centre. Signed-in tickets are attached to your account, which means support can see which account you are asking about without you sending anything sensitive.
What happens next
You get an email confirming the ticket, and another whenever support replies. Replying in the thread keeps everything in one place.
What not to send
Never send your password, API keys, or card details. Roundly support will never ask for them.