How to Create Custom Columns for Both Parent Items and Subitems on Monday.com via API

I need to create both parent items (items) and subitems with custom columns using the Monday.com API. While I have successfully been able to create custom columns for the parent items, I am encountering some difficulty when trying to create custom columns for the subitems.

using board id i will create column in item not for subitem then how to create subitem custum column