You can only open one 1 Year Fixed Rate ISA with Moneybox. Once you open your account, a 7 day funding window begins – any money you pay in during this window will be held in Vaults for the duration of your 1 year term, earning a fixed interest rate.
Once your first funding window ends, you can continue to pay in by opening a new funding window, if one is available. There’s no limit to the number of funding windows you can open, so you can pay into your 1 Year Fixed Rate ISA for as long as you’d like.