It's great to have you join us, and thank you for sharing your question about what to put underneath your fake grass.
It's great that you've received excellent advice from @MikeTNZ. If this is the first time you've laid artificial turf, I recommend thinking about drainage and water flow. If your area is not prepared properly it might cause unintended water pooling or water flowing towards your house instead of the drain. We look forward to seeing the photos. If you have any other questions, please don't hesitate to post them. Our members would be more than happy to give you advice and information should you need it.