How to record and expense and Owner drawing in the same transaction

How to record and expense and Owner drawing in the same transaction

Good Morning,
I cannot work out how to allocate a bill to both an Expense account and a Drawing (Equity account) in the same transaction.  I have a home office and part of the expense is for the home office and part is for the home (not a business expense).  I cannot pick up an equity account  in Purchases.  I also cannot journal the non expense portion as I have to reduce the expense that includes gst. and the journal will not record as the other side of the entry (Equity) does not include gst so the debit and credit is not equal.