Purchase Invoice Must have Automatic Invoice Number
The reason purchase invoices do not have automatic numbering is because they record invoices sent to you by suppliers. Normal practice is to record the supplier’s invoice number so it can be referenced when you pay. Assigning your own number could create serious confusion.
Can any thing be created like ways in custom fields
Not with automatic numbering. Custom fields are for information you enter yourself.
Custom fields don’t support counters but they could in future which would be great for this use-case.