From where do we get to know the API names for the fields of different rows in a  subform. For example, if there is a subform called "Items" and it has fields "Item Name", "Item Price" and "Quantity", how can i assign the input value of these fields to CRM fields?

For example: I have fields in CRM as  "Item Name 1", "Item Price 1" and "Quantity 1,  "Item Name 2", "Item Price 2" and "Quantity 2",  "Item Name 3", "Item Price 3" and "Quantity 3",  "Item Name 4", "Item Price 4" and "Quantity 4"?

How can I push the values from the 4 rows of "Items" subform in creator to these CRM fields?