Save your API Key as a system environment variable CFBD_API_KEY

cfbd_key()

has_cfbd_key()

Details

To get access to an API key, follow the instructions at https://collegefootballdata.com/key

Using the key:
You can save the key for consistent usage by adding CFBD_API_KEY=XXXX-YOUR-API-KEY-HERE-XXXXX to your .Renviron file (easily accessed via usethis::edit_r_environ()).
Run usethis::edit_r_environ(), a new script will pop open named .Renviron, THEN
paste the following in the new script that pops up (without quotations)

CFBD_API_KEY = XXXX-YOUR-API-KEY-HERE-XXXXX

Save the script and restart your RStudio session, by clicking Session (in between Plots and Build) and click Restart R
(there also exists the shortcut Ctrl + Shift + F10 to restart your session).

If set correctly, from then on you should be able to use any of the cfbd_ functions without any other changes.

For less consistent usage:
At the beginning of every session or within an R environment, save your API key as the environment variable CFBD_API_KEY (with quotations) using a command like the following.

Sys.setenv(CFBD_API_KEY = "XXXX-YOUR-API-KEY-HERE-XXXXX")