Hiiii There
Your opening balance is your bank balance after the last reconciled transaction. You can adjust it by adding the transaction that’s causing an overdraft. You can add it within the register as an expense. Then, you can mark it as Reconciled (R). It’s the R mark that triggers it.
By the way, the overdraft happens in July first and the month isn’t over yet. If you haven’t reconciled in July, you won’t want to mark that transaction as reconciled to keep your opening balance correct. You’ll want to leave the Reconcile (check mark) field as blank or Cleared (C) instead.