"HTTP API sources in EU workspaces should use the events.eu1.segmentapis.com endpoint
If you are located in the EU and use the https://api.segment.io/v1/ endpoint, you might not see any errors, but your events will not appear in the Segment app. For more information about regional support, see the Source Regional support documentation."
Thank you for posting your question. I’ve checked on our side, and currently, it’s not possible to configure an in-house segment connector to use the EU URL to receive data. However, you should be able to configure the Custom Webhook to send data to the proper endpoint. I can’t test for the EU workspace, but I’ve managed to set up the Custom Webhook to send data to the US workspace with Basic Authentication → HTTP API Source | Segment Documentation
Unfortunately, it’s not possible to modify the format of the JSON body. But I encourage you to write up a product request in the Feedback category. You can read our FAQ regarding submitting a feature request here: How do I submit Product feedback or feature requests?