JSFiddle - React, Tailwind, and code Playground

HTML

<form action="https://external.com/login" method="POST" onsubmit="return postJson(this)">
    <input name="username" value="user" />
    <input name="password" value="12345" />
    <input type="submit" />
</form>

JavaScript

function postJson(form) {
    var action = $(form).attr('action');
    var user = $('[name="username"]', form).val();
    var pass = $('[name="password"]', form).val();
    
    console.log('action=' + action + ', user=' + user + ', pass=' + pass);
    
    $.ajax({
        url: action,
        type: 'POST',
        data: JSON.stringify({ username: user, password: pass }),
        success: function(data, status) {
            // do something after login
            console.log('success');
        },
        error: function() {
            console.log('error');
        }
    });

    return false;
}