How can you assign an expense to a customer in an invoice. I have Essentials subscription

Step 1 is to turn "on" make expenses billable from your company settings (gear icon) once you do so two extra columns should appear on invoices and checks. 

Step 2 is to enter customer/job name in the line item of expense and then verify thatbix to bill ischecked.

Step 3 is to create an invoice or sales receipt for the customer and agree to add time/expenses.

At this point you can add a markup or not

If you cannot get past step 1 then step A will be to upgrade to Plus

@j-goodney 

"How can you assign an expense to a customer in an invoice. I have Essentials subscription"

To assign an expense to a customer in an invoice, you will need a Plus plan or Advanced plan. In Essentials plan (or Simple Start plan) such a feature in QBO is not available at this time. You can check out this helpful article to find out available features in different QBO plans.