In QBO, they have “Pre-fill forms with previously entered content” rather than specifically selected expense accounts to Pre-fill like in QB desktop version.

From home page screen, select Company Gear (top upper corner)
Select Company Settings under Settings
Select Advanced
Select Automation
Select Pre-fill forms with previously entered content
Double click to edit “Pre-fill forms with previously entered content” from “Off” to “On”
Click Save and Done
They have different QBO versions floating around, your selections may vary slightly.

See attached screenshot below for reference.