How do I create a javascript bookmark that fills in user and password fields?

javascript:document.body.contentEditable = 'true'; document.designMode='on'; void 0

Saving this line of javascript code allows you to edit the text content of a website once you click on the bookmark.

My question is what code can I save with predefined values to be inserted in a Username<input class="config" type="text" name="username" id="username"> and Password<input class="config" type="password" name="password" id="password"> field.

I can't save it in my browser because the url address is different with every IP address we access.

I want to save a bookmark with a line of code as mentioned above that already has the user and password values saved as admin and 12345 for example. Then instead of entering it by hand I can just click on the saved bookmark and it is entered.

Thank you to everyone who tries helping or replies.