How do I get the hidden badge on Steam for the SteamSale ARG?

1. Go to http://store.steampowered.com/app/253750 in your normal browser (you will need to log into Steam first if you are not already)

2. Wait until the site has been fully loaded

3. Type search on your Keyboard then press ENTER key

4. In the new popup window, enter the code 1v7531

This is how it looks like:

Remark for 3: Don't click into the search field, just type it directly after the site has loaded without hovering or clicking anything.

After enterting this, you will be redirected to a page showing your new badge. The badge is also featureable on your profile. This does not work within the steam client.

Make sure you are logged into Steam while doing this, otherwise you will get an error. Thanks to Gauzy for the reminder.

In case the other answer doesn't work, you can open the game Ikaruga's page through a browser, and run this code in the javascript console:

This will simulate the process of doing it:

$J.ajax({
    url: "http:\/\/store.steampowered.com\/actions\/clues",
    dataType: "json",
    cache: false,
    data: {key: "1v7531"}
}).done(function( result ) {
    if( result && result.url )
        window.location = result.url;
    else if( result.response )
        alert(result.response);
});

Source: YanirX on Pastebin