add_action('save_post', 'save_my_post_meta');

function save_my_post_meta($post_id) {
    // Bail if we're doing an auto save
    if( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) return;

    // if our current user can't edit this post, bail
    if( !current_user_can( 'edit_post' ) ) return;

    // now we can actually save the data
    $allowed = array(
        'a' => array( // on allow a tags
            'href' => array() // and those anchors can only have href attribute
        )
    );
    // If any value present in input field, then update the post meta
    if(isset($_POST['mytext'])) {
        // $post_id, $meta_key, $meta_value
        update_post_meta( $post_id, 'mytext', $_POST['mytext'] );
    }
}