Good Evening, @evp1.
I hope your day is going well so far. To properly assist you with this issue, are you wanting to transfer your QuickBooks Desktop (QBDT) Pro 2018 to your QuickBooks Online (QBO) account?
If not, please elaborate on what you mean so that I can determine the best solution for you and your business. If so, then you can transfer your company data, but the information within your QuickBooks that already exists will be overwritten by the new data importing over.
Exporting your data from QBDT to QBO is a simple process. Here's an article that can help you out: Move your QuickBooks Desktop file to QuickBooks Online.
If there's anything else I can help you with, feel free to ask. I'm always here to lend a helping hand.