What Tax Code do I use for University Courses booked from the UK

[Nick Brown]

Good morning community, I am trying to register and pay for a Atlassian University Course from here in the UK but when I complete the online payment form it asks for my Tax Code, I tried using my National insurance number but that fails. Anyone help with what needs to go in this field.

I am unable to book that courses I need without being able to get past this step?

Thanks