curl --request POST \
  --url https://api.zenskar.com/customers/{customer_id}/relationships \
  --header 'Content-Type: application/json' \
  --header 'organisation: <api-key>' \
  --header 'x-api-key: <api-key>' \
  --data '
{
  "related_customer_id": "3c90c3cc-0d44-4b50-8888-8dd25736052a",
  "relationship_type_id": "3c90c3cc-0d44-4b50-8888-8dd25736052a"
}
'

{
  "id": "3c90c3cc-0d44-4b50-8888-8dd25736052a",
  "customer_id": "3c90c3cc-0d44-4b50-8888-8dd25736052a",
  "related_customer_id": "3c90c3cc-0d44-4b50-8888-8dd25736052a",
  "relationship_type_id": "3c90c3cc-0d44-4b50-8888-8dd25736052a"
}