Using rvest or httr to log in to non-standard form

I am attempting to use rvest to spider a webpage that requires an email/password login on a form.

rm(list=ls())
library(rvest)

### Trying to sign into a form using email/password 

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

set_values(pgform, `ctl00$Header2$HeaderTop1$tbUsername` = "myemail@gmail.com") 
set_values(pgform, `ctl00$Header2$HeaderTop1$tbPassword` = "mypassword")

submit_form(pgsession,pgform,submit=`ctl00$Header2$HeaderTop1$Button1`)

This gives me the following error message:

Error in submit_request(form, submit) :

object 'ctl00$Header2$HeaderTop1$Button1' not found

If I submit the form without specifying the submit parameter, I get this:

Submitting with 'ctl00$Header2$HeaderTop1$Button1'
Error in function (type, msg, asError = TRUE)  : <url> malformed

I also tried passing the parameters directly to httr as mentioned in this question: How can I POST a simple HTML form in R?, but the "submit" parameter did not accept the submit button either with backwards quotes (``), quotation marks, or without any quotes:

library(httr)

url <- "http://www.perfectgame.org/Rankings/Players/Default.aspx?gyear=2015&num=500"

fd <- list(
    submit = `ctl00$Header2$HeaderTop1$Button1`,
    `ctl00$Header2$HeaderTop1$tbUsername`  = "myemail@gmail.com",
    `ctl00$Header2$HeaderTop1$tbPassword`  = "mypassword")

resp<-POST(url, body=fd, encode="form")
content(resp)

Any ideas for how I can log in from an R session and spider the data that's behind the login wall?

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$tbUsername` = "myemail@gmail.com", 
  `ctl00$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).