If you're a student who is currently using Zapier on your own and you want to add custom fields internally so you can add more information, it'll be accessible, but if you're a client and you also want to add a field, make sure to watch this video to be certain with what you will do. https://youtu.be/SePOXdl4PBU
Note that in most cases, do use "Text" or the "Multi-line text" fields so you can add any other information you want to be stored in your field without restrictions.