I think Thunkable uses their own public key by default, that’s why there’s no option in neither designer, nor in blocks to set your custom/own API key 
Thanks! 
I think this is not so.
Hi there,
We are working on a fix to this at the moment.
I will reply to this post again when this issue has been resolved.
Jane
Hi again,
This has now been fixed! You should be able to see these settings in the Properties window of your Assistant component.
Jane