You get the kit for the test directly from the UK and return it there but your vet could undoubtedly help with this.
However -- dogs with advanced heart disease also can have collapsing episodes. This would look quite like an EFS episode as they often remain aware but fall over and slowly recover. So it would be good to discuss with a cardiologist as well as having the EFS DNS test (which would be conclusive). It would really be advisable to see a cardiologist for a dog with a murmur anyway.
Cavaliers: Tansy : Mindy Connie Roxy Neasa Gus
In memory: My beautiful Jaspar Lucy Leo Lily Libby