You don't/can't, at least as far as I'm aware. All our VCF deployments (inc. NSX Federated ones) we manage as separate entities. I think the VCF federation that was deprecated caused a lot of issues (and also I guess limited your WLD's if all the vCenters across multiple sites were in ELM).
In reality it's not really an issue for us, I only really use SDDC Manager for upgrades and password/cert changes so having to do that in multiple different ones isn't an issue as each VCF deployment has different expirations and upgrade cycles anyway.
VC ELM across NSX federated VCF deployments would be nice but I'm just used to having separate browser tabs for each site's VC ELM
