I don’t think an NIF is required, only your ID such as a passport.
The requirements shown in their application are:
- Address proof (e.g. electricity, water, gas, driving license, credit or rental agreements)
- Proof of profession (e.g. invoice of work as self-employed person, statement of start of activity or last personal income tax return)
I went through the signup process on their app but didn’t complete it since I currently don’t have any proof of profession to show.