Error Subitem cannot be empty in Purchase Order

When i try to create an Purchase Order using the REST API in the postman i am getting an "error": "'SubItemID' cannot be empty.; SubItemID: 'SubItemID' cannot be empty." How can i resolve this issue ? from where will i pass the SubItemID is it in the vendors , stock items or some other tab ? also where will i send in my Products ? How can i do it ? 
This is what i am sending in the Request body 
 

This is the response i am getting 
 

Also if i need to send the list of products where should i add it and where will it be seen ??