Using rvest or httr to log in to non-standard forms on a webpage

Your rvest code isn't storing the modified form, so in you're example you're just submitting the original pgform without the values being filled out. Try:

library(rvest)

url       <-"http://www.perfectgame.org/"   ## page to spider
pgsession <-html_session(url)               ## create session
pgform    <-html_form(pgsession)[[1]]       ## pull form from session

# Note the new variable assignment 

filled_form <- set_values(pgform,
  `ctl00$Header2$HeaderTop1$tbUsername` = "[email protected]", 
  `ctl00$Header2$HeaderTop1$tbPassword` = "mypassword")

submit_form(pgsession,filled_form)

And I now see a nice 200 status code response instead of an error. Note that because the desired submit button appears to be the first submit button, we don't need to give it as an argument, but otherwise we'd just be giving it a a string (straight quotes, not back quotes).