Thanks for getting in touch with the Community, THassellPeck.
If you’ve already created an account without an opening balance and want to add one, you can add an opening balance later with a journal entry.
Before doing so, check your account register and confirm you don't already have an opening balance. Find the account and click View register from its Action column, then search for an opening balance entry. In your Payee Account column, it should say Opening Balance Equity, and in the Memo column it should say Opening Balance.
If you see an opening balance entry, don't go further. Take note of the date and amount. Use your bank statements to make sure the opening balance is correct. In the event you don't see an opening balance, write down the date and amount of the oldest transaction in the account.
Next, you can create a journal entry to record your opening balance if necessary.
Please feel welcome to send a reply if there's any additional questions. Have a great Monday!