Register Key(s) Page
The Steam Register Key page (available only if you're logged in) offers both an input field to enter a single key and an "Activate Multiple Products" button, opening a new input field in which you can enter multiple keys, one per line.
The key input field can be auto-filled by by appending a
key= parameter to the URL, e.g.,
However, this is not entirly automatic; the user will still need to click the "Continue" button in the page brough up by this URL.
If you're redeeming Humble Bundle keys, exposing the key now shows both the key and a button that will open the above URL in a new tab, so you need only click that and "Continue" for each key.
The above URL can also be used for multiple product codes, though it's not quite as user-friendly. Concatenate the multiple keys in the
key= parameter with a comma between them, e.g.
key=XXXXX-XXXXX-XXXXX,YYYYY-YYYYY-YYYY. This will fill the single-key input box with the comma-separated keys. This cannot be accepted as a submission ("Continue" will produce an error), but if you then push the "Activate Multiple Products" button you'll find the full contents of that field in the multi-line input box with each value on its own line, and you can submit that in the normal way by clicking "Activate Products."