Hello there, scenicescapest. You're unable to set your own rate as the permanent default in QuickBooks Online (QBO). The system first pulls the current market rate. What you can do is manually edit the rate directly into the single transaction.
For multiple foreign currency transactions, you can update the rate in your currencies list, and QBO will use it for all new transactions you create that day. To do this, go to Settings ⚙ > Account and settings > Advanced, then in the Currency section, select Manage Currencies. Find the currency you want to update, choose Edit currency exchange under the Action column, enter your preferred rate, and click Save.
Please note that this rate only last for the day. The next day, QBO will revert to the market rate, so you'll need to update it again if needed.