silverstripe-gridfieldextensions/code/GridFieldAddNewMultiClassHandler.php

<?php
/**
 * A custom grid field request handler that allows interacting with form fields when adding records.
 */
class GridFieldAddNewMultiClassHandler extends GridFieldDetailForm_ItemRequest {

	public function Link($action = null) {
		if($this->record->ID) {
			return parent::Link($action);
		} else {
			return Controller::join_links(
				$this->gridField->Link(), 'add-multi-class', $this->sanitiseClassName(get_class($this->record))
			);
		}
	}

	/**
     * Sanitise a model class' name for inclusion in a link
     * @return string
     */
    protected function sanitiseClassName($class) {
        return str_replace('\\', '-', $class);
    }
}