After you've click on Create in the listing you'll see this page.
As you can see several actions that were on the listing are there to:
But there is other information to fill, and the first one is the name of you custom field the only one required. All the information that you see after Editable on user modification will depend on the type of field you've choose.
Here are the different fields type:
We will now add a Gender field as our goal is to include in our Newsletter "Hello Mr John..."
These are the information to fill:
Field type: Single Dropdown
We will make it a require field so if the user didn't select any value, an error message will be displayed
We add a series of values
Note that you can drag and drop the values like in the listing.
We will create a birthday field so you can send an email on the birthday of your users
The Field type to choose is Date
We'll make it required to make sure to have this information to send birthday email.
You can customize the format of your date field, for example %d%m%y will display 01/01/2019