Hi there, BenB90.

You can only assign a Class when creating journal entry in QuickBooks Online. Adding a Project field is currently unavailable. 

 Although, you can associate it to a customer or sub-customer. Just make sure your Class Tracking feature is turned on for the Class column to show up.

Please see these support articles for more details:

• Set up and use class and location tracking
• Create a journal entry

Please click the Reply button below if you have follow-up questions.