The minimum required fields to create a lead are:
1.- Form of contact
2.- First name
3.- Last name
4.- Phone
5.- An email
The rest of the information is optional.
TIP: If your client does not have an email, you can register your own email or a placeholder email. Example: name@nodomain.com. The same applies to the phone, you can add your office phone or a non-existent number like 11111