Should you season the meat of a patty before or after it's formed? I've heard that it allows for more flavour if you were to add salt and pepper from the start, but I'm told doing that will cause the patty to fall apart more likely into just ground beef and that you should always season it AFTER the patty is formed.
Kenji Lopez-Alt wrote about this issue on the website, Serious Eats. Salting the meat before making the patty does not cause it to fall apart, rather it creates a more bouncy, sausage-like texture. His conclusion is that most people prefer a patty that has a loose, open structure. His advice is to salt immediately before cooking.
I won't talk about salt as I cannot add to @moscafj's answer. However, pepper needs to be mentioned because it is different than salt. Pepper tends to get bitter under direct high heat, so it should be either mixed in before forming or added just after cooking. When mixed in the meat will protect the pepper from being torched, if you pepper right after cooking the residual heat and the fats will extract good pepper flavor.