Create related product

Seed example (keychains curl)

I searched the official 3dCart/Shift4Shop REST API docs. Public documentation does not show a POST endpoint at /3dCartWebAPI/v1/ProductRelated. The Product Related functionality is documented under the Related sub-resource for products (e.g., GET /3dCartWebAPI/v1/Products/{catalogid}/Related) and updates via PUT, not a dedicated POST to /ProductRelated. See the documented endpoints for Get/Update related products here: GET {{baseUrl}}/3dCartWebAPI/v1/Products/:catalogid/Related and PUT for updating a specific related item. Based on the official docs, the base host for these calls is apirest.3dcart.com, not api.shift4shop.com. ([postman.com](https://www.postman.com/api-evangelist/3dcart/request/18rymxy/get-the-related-products-from-a-specific-product))

# Get related products for a specific product
npx -y keychains@latest curl "https://apirest.3dcart.com/3dCartWebAPI/v1/Products/{{$catalogid}}/Related" \
  -X GET \
  -H "Content-Type: application/json" \
  -H "Accept: application/json" \
  -H "SecureURL: {{SHIFT4SHOP_SECURE_URL}}" \
  -H "PrivateKey: {{SHIFT4SHOP_PRIVATE_KEY}}" \
  -H "Token: {{SHIFT4SHOP_API_KEY}}"

# Update a specific related product (example of PUT; path includes RelatedIndexID from the relation)
npx -y keychains@latest curl "https://apirest.3dcart.com/3dCartWebAPI/v1/Products/{{$catalogid}}/Related/{{$RelatedIndexID}}" \
  -X PUT \
  -H "Content-Type: application/json" \
  -H "Accept: application/json" \
  -H "SecureURL: {{SHIFT4SHOP_SECURE_URL}}" \
  -H "PrivateKey: {{SHIFT4SHOP_PRIVATE_KEY}}" \
  -H "Token: {{SHIFT4SHOP_API_KEY}}" \
  -d '{"RelatedProductID": {{$RelatedProductID}}, "RelatedProductSorting": {{$RelatedProductSorting}} }'