Using Windcave to process donations

I am in the process of switching over our charity website from Silverstrip to Squarespace. However we currently use Windcave to process donations and there is little appetite to change this... I have tried. 

What are my options here for creating a donation page using Windcave and embedding it in the Squarespace site? I'm happy to hire a developer for this part of it if needed.

This is our current donation page https://www.cholmondeley.org.nz/support-us/donate/all I need to do is replicate this.