aws-cdkでの環境構築メモです。
特定のBucketへのPutをトリガーに発火するLambdaを作っています。
ただし、Lambdaのコードベースはcdk管理外のS3で管理されているものとします。

1. fromCfnParametersのdefault値で無理やり当てる

...
    /* S3  */
    const originalVideoBucket = new Bucket(this, `OriginalVideoBucket` , {
      bucketName: resources.originalVideoS3Bucket.name,
      removalPolicy: cdk.RemovalPolicy.DESTROY, // RETAIN
      versioned: true,
      publicReadAccess: false,
    });

    const fn = new lambda.Function(this, 'OriginalVideoAssetUploaded', {
      functionName: resources.originalVideoUploaded.lambdaName, 
      runtime: lambda.Runtime.GO_1_X,
      handler: 'main',
      memorySize: 128,
      code: lambda.Code.fromCfnParameters({
        bucketNameParam: new CfnParameter(this, "OriginalVideoBucketLambdaSourceBucket" , {
          default: <既存のS3Bucket>,
        }),
        objectKeyParam: new CfnParameter(this, "OriginalVideoBucketLambdaSourceS3Key" , {
           default: <既存のS3Key>,
        }),
      }),
    });

    fn.addEventSource(new  eventSource.S3EventSource(originalVideoBucket , {
      events: [ EventType.OBJECT_CREATED],
    }))

2. Bucket.fromBucketName

    const resourceBucketRef = Bucket.fromBucketName(this , `resource` , resources.resourceS3Bucket.name);

    const regionalUserApiLambda = new lambda.Function(this, 'RegionalUserApi', {
      functionName: resources.userApi.lambdaName,
      runtime: lambda.Runtime.GO_1_X,
      handler: 'main',
      memorySize: 128,
      timeout :  Duration.seconds(10),
      code: lambda.Code.fromBucket(resourceBucketRef , resources.userApi.lambdaS3Key)
    });

3. `@aws-cdk/aws-sam` の方のCfnFunctionを使う

import * as Sam from "@aws-cdk/aws-sam"
....
....

    const samFunction = new Sam.CfnFunction(this, 'OriginalVideoAssetUploaded', {
      functionName: resources.originalVideoUploaded.lambdaName,
      codeUri: {
        bucket : <既存のS3Bucket>,
        key: <既存のS3Key>
      },
      handler: 'main',
      runtime: 'go1.x',
      memorySize: 128,
      timeout: 3,
      events: {
        "mys3event": {
          type : `S3`,
          properties : {
            bucket : originalVideoBucket.bucketName,
            events : `s3:ObjectCreated:*`,
          },
        },

      },
    });

こっちのパターンはs3 eventの方がうまく設定されないのでちょっとCloudformationとしての構文出力と照らし合わせて検証中です