AWS: createUserPool: How to set up SmsConfiguration (ExternalId and SnsCallerArn)?

I want to create a userPool for every enterprise within a multi-tenant web solution. Here's what I have:

const AWS = require('aws-sdk');
const uuid = require('uuid');
const cognitoIdentityServiceProvider = new AWS.CognitoIdentityServiceProvider();

params: {
   PoolName: uuid.v4();
   ...
   SmsConfiguration:  {
      ExternalId: 'some Id',
      SnsCallerArn: 'some arn:aws:iam'
   }
   ...

My attempts to apply what I think is the correct policy to an existing role attached to my lambda causes the iam:PassRole exception to occur. How then and where am I supposed to create the proper ExternalId and SnsCallerArn? Thanks.