Add something after a filter

that’s exactly what the filter is here for! apply_filters is a hook that allows you to modify the array that is passed there as a second argument. Try this:

function example_additional_wpneo_menus( $menus ) {

    // check the type just in case
    if ( ! is_array( $menus ) ) {
        return $menus;
    }

    $menus['myform'] = [
        'tab'            => 'myform',
        // here the second arg is a name of your theme/plugin textdomain, used for translations only
        'tab_name'       => __('My Form','your_textodomain'),
        // MY_THEME_DIR doesn't seem to be a wordpress constant, try this if you want to get current theme directory
        // https://developer.wordpress.org/reference/functions/get_template_directory/
        'load_form_file' => get_template_directory() . '/wpcrowdfunding/dashboard/myform.php'
    ];

    return $menus;
}

// now you hook that into the filter
add_filter( 'wpneo_crowdfunding_frontend_dashboard_menus', 'example_additional_wpneo_menus', 10, 1 );