Hi there, sam49.
You can use the Notes field to indicate the lead source when setting up the project in QuickBooks Online. You can also use the same field for all other information related to the project.
However, you'll need to manually open the project to see what the lead was. We're not able to search details from the Notes field using the Search menu.
I'll send a feature request about adding a field for project leads in the Project Center. They review customer feedback and suggestions for upcoming updates.
Let me know if you need anything else.