<form action="<?php echo admin_url('admin-post.php') ;?>" method="POST">
    <input type="hidden" name="action" value="salva_impostazioni_social">

    <div class="container-fluid">
        <h4 class="mt-3">Social</h4>

        <h6 class="m-0">Twitter</h6>
        <div class="row mb-3 mt-3">
            <div class="col-12">

                <!-- USERNAME TWITTER -->
                <div class="row">
                    <div class="col-4">
                        <div class="input-group mb-3">
                            <div class="input-group-prepend">
                                <span class="input-group-text" id="twitter-username">@</span>
                            </div>
                            <input type="text" name="twitter_username" value="<?php echo $twitter_username; ?>" class="form-control" placeholder="Twitter Username" aria-label="Username" aria-describedby="twitter-username">
                        </div>
                    </div>
                </div>

                
            </div>    
        </div>

        <hr>

        <div class="row mb-3">
            <div class="col-12">
                <button class="btn btn-primary">Salva</button>
            </div>
        </div>
    </div>
</form>