Customers Keywords Plan API Trouble

[Dev Development]

Hi there,

We've been having trouble for months now getting a resolution on our API integration with the keywords planner api.

This is our request:

{
"path": "https://googleads.googleapis.com/v13/customers/8674944107/keywordPlans:mutate",
"args": {
"method": "POST",
"body": "{\"operations\":[{\"create\":{\"forecastPeriod\":{\"dateInterval\":\"NEXT_MONTH\"},\"name\":\"eddlwx3vflycujbpjbemo4t6\"}}]}",
"headers": {
"Content-Type": "application/json",
"Accept": "application/json",
"developer-token": "REDACTED",
"login-customer-id": "REDACTED",
"Authorization": "REDACTED"
}
}
}

You’ll see login-customer-id is set to REDACTED.

But there’s another parameter that is set, called customerId, which is set to 8674944107. https://googleads.googleapis.com/v13/customers/{customerId}/keywordPlans:mutate

This is the error I'm getting:

User doesn't have permission to access customer. Note: If you're accessing a client customer, the manager's customer id must be set in the ‘login-customer-id' header. See https://developers.google.com/google-ads/api/docs/concepts/call-structure#cid
I'm unsure where we get the customerId from?? (This is not the login-customer-id)