Message request is the most common way to contact other users.
If you just need to ask someone any information you can submit a message request via button "Book now" (sending an inquiry) via profile. But it's not over, other user then receives a message request and then he/she can accept or reject it. If user accepts your messsage request then your message goes to your inbox (messages) where you both continue the conversation.
Read How to view received and sent message requests for more information.
