I'm managing a REST API for an app with Serverless and want to extend this setup with a WebSockets API in the same region. Everything should be handled with the same certificate, but different subdomains.

At first I created a new custom domain with `sls create_domain --stage=...`.
Then I tried to add it to the new WebSockets stack, but ended with this error:

> 
> Error: Failed to find CloudFormation resources for ...
> 
> 
> 

I found it on Github that this seems to be not supported by CloudFormation right now so Serverless does not support it.

So I tried to attach my stage to the custom domain name manually in the UI:

> 
> Mixing of REST APIs and HTTP APIs on the same domain name can only be
>  accomplished through API Gateway's V2 DomainName interface. Currently,
>  WebSocket APIs can only be attached to a domain name with other
>  WebSocket APIs. This must also occur through API Gateway's V2
>  DomainName interface.
> 
> 
> 

More confusion arises, as it's not even the same domain name in this case. The new domain name was `sockets.<DOMAIN>.com` and the existing one was `api.<DOMAIN>.com`. Or do different subdomains are falling into 'same domain name'?

Nevertheless I tried to create the custom domain again via the apigatewayv2 CLI:

```
aws apigatewayv2 create-domain-name --domain-name <DOMAIN> --domain-name-configurations file://domain-configuration.json --region eu-west-1

```

domain-configuration.json:

```
[
{
    "ApiGatewayDomainName": "<DOMAIN>",
    "CertificateArn": "arn:aws:acm:us-east-1:<ACCOUNT_ID>:certificate/<CERT_ID>",
    "CertificateName": "<DOMAIN>",
    "DomainNameStatus": "AVAILABLE",
    "EndpointType": "EDGE",
    "SecurityPolicy": "TLS_1_2"
}

```

]

But this results in the following error:

> 
> An error occurred (BadRequestException) when calling the CreateDomainName operation: Invalid certificate ARN: arn:aws:acm:us-east-1:924441585974:certificate/b88f0a3f-1393-4a16-a876-9830852b5207. Certificate must be in 'eu-west-1'.
> 
> 
> 

My current state was that API Gateway only allows custom certificates to be located in us-east-1, so this error confuses me even more.

**Summary**: I'm completely stuck on how to get a custom domain name attached to my WebSocket API stage. I'm happy about every hint in the right direction!