How to auto-populate the lookup field depends on the selected- lookup

I try using the workflow but it need to save the transaction before it populate, I try mapping it but doesn't work.

So I have a 3 Entities Expense,Vendor,Transaction.

In Vendor Entity I have a field of Vendor Name and a lookup field for Expense Category(Expense Entity).

In Transaction Entity I have a lookup for Vendor name and Expense Category, so when I select a vendor name in the lookup it will populate the lookup of expense category.

I already used the filter, yes it show the expense category depends on the vendor name selected, but the problem in there is I need to click the lookup field of expense tracker to show the value. I want is it will auto fill the lookup field without clicking it. Thank you