I have never gotten testing from RAL for Mareks, but several people here have posted about trimming a toenail a little short where a few drops of blood can be collected. I’m not sure of the medium they use, whether it is collected in a tube or on a card or what. I think @azygous was going to do that a few months back. She might be able to tell us. But you can contact the lab by phone and ask how it is collected. Or ask your vet if the toenail trimming won’t work. Here is the link:
https://www.vetdna.com/application/forms/aviansubmittalform.pdf
https://www.vetdna.com/application/forms/aviansubmittalform.pdf