Untitled

var myButton = document.getElementsByTagName("input")[2];
var myForm = document.getElementsByTagName("form")[0];

myButton.onclick = function(){
    if(myForm.username.value == "root"){
        if(myForm.password.value == "1q2w3e4r5t"){
            console.log("you did it!");
            window.location = "https://www.google.co.uk/";
            alert("Login is successful!");
        }
        else{
            alert("Login credentials are wrong try again!");
        }
    }
    else{
        alert("Login credentials are wrong try again!");
    }
}


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<!DOCTYPE html>
<html lang="en">

    <head>
        <meta charset="utf-8">
        <title>Example2</title>
    </head>
    <body>
        <form align="center" action="">
            <br>
            Username:<input type="username" name="username"><br><br><br>
            Password :<input type="password" name="password"><br><br>

            <input type="submit"  value="Submit"/>
            <script src="login.js"></script>
        </form>
    </body>
</html>