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