There is no username or password required to access the Custom Fields application. You just have to log in from your Shopify store.
Step 1: Click on Apps
Step 2: Click the Custom Fields Icon
The application will open in a new tab.
If you've followed the instructions above and are still having trouble logging in, check your browser settings and make sure your browser is set to "Accept 3rd Party Cookies".
Instructions to adjust this browser setting vary per browser.
Accept 3rd Party Cookies In Chrome
To fix this, open Chrome preferences click on Settings, then Show Advanced Settings. Under Privacy, click on Content Settings. Make sure "Block third-party cookies and site data" is not checked.