Only the Account Admin is able to manage that. If you do not see the following options, you’re not the Account Admin.
To add a new custom field, click
Account & Settings
button across the top of your account, then click
Next, on Custom Fields page, select the document for which you’d like a custom
field, then click the
On the New custom field page fill out the appropriate information and scroll
down to click
Create custom field button to add this field to the respective
You have successfully added the new custom field to your document.
And you are done!