Use ${ServerlessHttpApi} in AWS SecretManager SecretString

Question

How to populate the Secret Manager Notification Url with the API URL? The code below throws a circular reference. I have tried multiple !Sub but nothing seems to work

Resources:  
  SquareWebhookAuthorizerFunction:
    Type: AWS::Serverless::Function # More info about Function Resource: https://github.com/awslabs/serverless-application-model/blob/master/versions/2016-10-31.md#awsserverlessfunction
    Properties:
      CodeUri: square-webhook-authorizer/
      Handler: app.SquareWebhookAuthorizerFunction
      Runtime: nodejs16.x
      Environment:
        Variables:
          SignatureKey: !Sub '{{resolve:secretsmanager:${SquareSecret}:SecretString:SignatureKey}}'
          NotificationUrl: !Sub '{{resolve:secretsmanager:${SquareSecret}:SecretString:NotificationUrl}}'
          TABLE_NAME: !Ref SquareWebhookEventTable
      Events:
        SquareWebhookEvent:
          Type: HttpApi # More info about API Event Source: https://github.com/awslabs/serverless-application-model/blob/master/versions/2016-10-31.md#api
          Properties:
            Path: /
            Method: POST

  SquareSecret:
    Type: AWS::SecretsManager::Secret
    Properties:
      Name: SquareSecretProxy
      Description: "Square App Secrets"
      SecretString: !Sub |
        '{
          "SignatureKey": "xxxx",
          "NotificationUrl": https://${ServerlessHttpApi}.execute-api.${AWS::Region}.amazonaws.com/",
        }'