What you will need to tell the bank is whether you want the overpayment to reduce the monthly payments or the term of the mortgage. It’s generally better to opt for reducing your monthly payment as it leaves you more flexibility.
I would also verify when you make the overpayment that it is fully applied. A couple of years ago KBC only applied part of an overpayment i made. I was doing €5k per day transfers which apparently confused things for them.