If you have more than one custom field to add, as Administrator you can upload custom fields through a CSV template.
Here are the steps to create custom fields through a CSV template.
1. Go to Admin > Custom Field > Select, Upload Custom Fields.
2. Click Download it here.
3. Fill in all the necessary fields. Once done, save the template into your desktop, then click Upload File.
Name Is the Custom Field Name
Enable Enter 1 = Active or 0 = Inactive
To add a custom field manually see article: