Follow these steps to add a custom field on the Account object.
Required Editions
- From Setup, go to the Object Manager tab.
- From the list of objects, select Account.
- From the left pane, select Fields & Relationships.
- Click New.
- Select Text as the data type, then click Next.
- Enter EmployeeNumber as Field Label and Field Name.
- Select the External ID checkbox, then click Next.
- Click Next and save your changes.