Both times I have entered Thailand in the past two years, it has been directly with the hotel. Major chains (like those in the Marriott portfolio) tend to be responsive about the booking requirements and emailed me afterwards requesting all the documents and sending the required documents for the Thai Pass system fairly quickly. Be careful of booking on Agoda and Booking since it seems many people get confused and select a regular room rate instead of the required Test & Go packages. If you have a contact directly at the hotel you are planning to stay at, that obviously helps as well.