Prazipro is made by Hikari. You may have heard of Hikari as the manufacturer of dozens of different fish foods. That is their specialty.
Prazipro was originally made for use in ponds (freshwater). In aquariums it was particularly good for curing Discus (freshwater). It is also sold for use in saltwater aquariums. I think its saltwater use is sort of a fifth wheel use of this product. Prazipro contains Praziquantel, as BTTRFLYGRL has posted. However, it contains less than 5% Praziquantel with no guarantee of it containing any measurable quantity of Praziquantel.
Next, it is a water treatment. For attacking intestinal parasites it is best to send the medication to the parasite in the gut vis a vis feeding it to them.
Lastly, I'm unsure how the fish will react to the other ingredients in Prazipro during their current treatment and how Formalin will react with the other ingredients. I am, however, sure of how the other food meds I've listed will work and that they are safe with your Formalin treatment. To use this product, you should get Hikari to say it is an appropriate treatment while you're performing a Brook treatment.
In my post I offer several other choices of medications that are fed to the fish. I would stick to a food additive or swallowed medication. Your LFS not having the others on my list, just go online and order some. There is time.