FindMyPast partner with LivingDNA, so you actually upload your Ancestry test to LivingDNA. You have to create an account with LivingDNA but there is no charge to create the account. Unlike Ancestry, your test isn't linked to your FindMyPast family tree, nor do you get match notifications through FindMyPast. You are able to view a list of matches via LivingDNA, but to find out any more about those matches, you have to contact them (and hope they reply). Because you are matching against other LivingDNA test kits, there is no indication whether your matches have trees on FindMyPast or not. Many of the kits will have been purchased through LivingDNA directly or uploaded from other test providers, so will have no connection with FindMyPast at all.
I'm not saying that it's not worth doing, but don't expect it to be as "easy" as finding and dealing with Ancestry DNA matches. The LivingDNA matches that I recognise in my own match list, I primarily know about because I have already matched them on Ancestry or via GEDmatch.