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.
Sample Template:
Name Is the Custom Field Name
Enable Enter 1 = Active or 0 = Inactive
To add a custom field manually see article: