AndiB

The domain ending ".squarespace.com" is built-in to your Squarespace website. If you cancel the web hosting subscription with Squarespace, the built-in Squarespace domain "authorsforvoicesofcolor.squarespace.com" will be deleted too.
If you are cancelling your Squarespace website, you'll probably want to try to forward your custom domain (authorsforblackvoices.org) to your page on the official charity auction site you will be using.

Please see the last bullet point of Before you begin in Forwarding a Squarespace domain.
It appears SS doesn't support what you want to do.
However. You could transfer your domain to a provider like GoDaddy and I think you'll find they have the feature you are looking for. Please do some research before you transfer to make sure the provider you choose can do what you want.