Guide or Documentation for Developing a SquareSpace Extension

Hi, I wonder if someone can help.

I am looking to develop a squarespace extension is there any documentation or guide available on how to get started?

Thanks for your help,

s7e

Hi,
If you haven't received an answer yet, you can post on some FB groups
+ Squarespace Customization Resource Groups
+ Squarespace Community
+ Squarespace Entrepreneur
+ Squarespace Rockstars

Hi, news? I need that guide too

I can see 2 parties using some unofficial api to create custom add on but they not willing to sell or share them.