To run some JS code as bookmark you need wrap your code in bookmark url as: javascript:(<here you code>)();
For example I generate GUUID this way:
javascript:(window.alert(([1e7]+-1e3+-4e3+-8e3+-1e11).replace(/[018]/g,c=>(c^crypto.getRandomValues(new Uint8Array(1))[0]&15 >> c/4).toString(16))))();
For several rows code use: javascript:(function(){<here you code>})();
javascript:(function(){document.querySelector("input[name='login']").value=my_login;document.querySelector("input[name='password']").value=my_pass;})();