If you see a home that you are interested in you should first send a contact request to the landlord. If the landlord approves your request, a notification will be sent to you and you will be able to chat. You can then can send a message to the landlord and request a viewing.
In other words, to arrange a viewing you and the landlord need to agree on a date and time.
Make sure to ask as many questions as possible before the viewing. This will hopefully make sure that you don't go on viewings that doesn't accommodate your needs.