New field on the POS screen

Add a new field to the POS screen, but I have the following problem: although I know when saving a transaction on the POS screen it becomes a bill of sale but when I consult this.frm.doc and this does not bring me the field added to sales invoice doctype

I am confused, what do I have to do to be able to access the field and make the mapping?

for newly added fields in standard doctypes, you will have to fetch that field from target doc once it is loaded (maybe there is a better, efficient way)