Adding note for supabase integration (#1101)

* Update management.mdx

Adding note for custom domains and initializing db object.

* Update management.mdx

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Co-authored-by: Matt Aitken <matt@mattaitken.com>

docs/integrations/apis/supabase/management.mdx

@@ -109,6 +109,9 @@ You'll have to do this for each Supabase project you want to use webhooks with.
 
 To use this feature, you'll first initialize a `db` instance, passing in your Supabase project [ID](https://supabase.com/dashboard/project/_/settings/api) (or URL):
 
+Note: If you're using a custom domain, you still need to use this format to intialize the db object:
+`https://<your project id>.supabase.co`.
+
 ```ts
 import { SupabaseManagement } from "@trigger.dev/supabase";