Magento 1.9.2.4 / PHP / Ajax Add to cart widget from pop up

General information for the website: Magento 1.9.2.4 / Magebird Popup / AJAX
Description of requirements/features: Platform & Dependencies:

Magento 1.9.2.4.

Athlete Theme (https://olegnax.com/product/athlete-responsive-magento-theme/livepreview/)

Magebird popup extension (https://www.magebird.com/magento-extensions/popup.html)

AJAX Add to cart

UI Requirements:

When a customer clicks 'Add to cart' from the product detail page; present a Magebird popup which includes a selection of Free Gift Products that can also be added.

The customer then clicks an 'Add to Cart' button for the Free Gift they want, and this is added to the cart along with the main product they added.

Technical Requirements:

A PHP & Client side script that adds a product to the cart, based on the ID or SKU passed to it from a parameter of attribute, and, also the ability to set a custom price as a parameter or attribute.

Ideally, either a Widget needs to be created that can be inserted in to a Magebird popup, or we need the ability to add a button / link with the required parameters, i.e.

ADD THIS FREE GIFT

This is needed ASAP!
Specific technologies required: Magento 1.9.2.4, PHP, JQuery, AJAX
Extra notes: