Yes, you can opt-in to the prize draw at any point.
For your entry to be valid for that month, the £10,000 minimum end-of-day balance is a requirement for the entire month, including all the days before you opted in.
If you didn't meet the balance requirement during the month, you will be automatically opted into the next month’s draw. This also applies if you’re a new customer joining Mettle part-way through the month.