CloudFormation template for API Gateway Hostname Discovery

template.js

"Resources": {
  "ApiGatewayRestApi": {
    "Type": "AWS::ApiGateway::RestApi",
    "Properties": { /* ... */ }
  },
  "ApiHostParam": {
    "Type": "AWS::SSM::Parameter",
    "Properties": {
      "Name": {
        "Fn::Sub": "/some/unique/path/probably/including/${App}/${Stage}/hostname",
      },
      "Type": "String",
      "Value": {
        "Fn::Sub": "${ApiGatewayRestApi}.execute-api.${AWS::Region}.amazonaws.com"
      }
    }
  }
}