I found the following code on w3schools:

var input = document.getElementById("passwordField");



    // Execute a function when the user releases a key on the keyboard

    input.addEventListener("keyup", function(event) {

        // Number 13 is the "Enter" key on the keyboard

        if (event.keyCode === 13) {

            // Cancel the default action, if needed

            event.preventDefault();

            // Trigger the button element with a click

            document.getElementById("loginButton").click();

        }

    });

Don't forget to specify the id's for the password field and login button, in this case 'passwordField' and 'loginButton'

click button when user clicks enter key in input field(s) ... or something similar