Currently, within the Create a New VPC Peering Connection : Request Body Parameters section of the Atlas documentation, the following required parameters are depicted:

Parameter	Required/Optional	Description
vpcId	Required.	The ID of the peer VPC.
awsAccountId	Required.	The AWS account ID of the owner of the peer VPC.
routeTableCidrBlock	Required.	The peer VPC CIDR block or subnet.

However, since the most recent Atlas release, an additional parameter is required when calling this API endpoint: containerId.

As an example, when attempting to call this API endpoint sans-containerId:

curl -X POST -u "$ATLAS_USERNAME:$ATLAS_API_KEY" --digest "https://cloud.mongodb.com/api/atlas/v1.0/groups/$ATLAS_PROJECT_ID/peers?pretty=true" \
 -H "Content-Type: application/json" --data '
 {
     "vpcId" : "$AWS_VPC_ID",
     "awsAccountId" : "$AWS_ACCOUNT_ID",
     "routeTableCidrBlock" : "10.0.128.0/17"
 }'

The following response is received:

{
  "detail" : "The required attribute containerId was not specified.",
  "error" : 400,
  "errorCode" : "MISSING_ATTRIBUTE",
  "parameters" : [ "containerId" ],
  "reason" : "Bad Request"
}

The containerId can be obtained via the Get a List of Cloud Services Containers API endpoint (returned as the id response element).

Ideally, the Create a New VPC Peering Connection : Request Body Parameters section of the Atlas documentation should be updated to include containerId as a required parameter.