You can communicate with the sender through Messages, located under the report. In case the sender did not disclose their name, the following conversation will remain anonymous from their side. It is also possible to send attachments through messages.
Messages can only be sent when the status of the report is Open. In case you want to write a message to the sender of a Resolved or an Archived report, you first need to move the report back to Open.
Tip: If you do not want your name to be displayed to the sender, you can hide it by ticking the box Do not display my name to the sender.