You need an active Premium Account to reply to a listing and message the landlord.
A Premium Account is a subscription (4 weeks) or a one-off (12 weeks) purchase that allows you to reply to listings.
After you have set up your subscription, you head to the listing you want to reply to and write a message in the text box at the bottom of the page.
When you've hit the 'Send message' button, it goes to the landlord!