Custom fields feature allow you to include additional information in your Projects. To enable this feature, click on your avatar in the upper-right corner and select the Account Settings option. Then navigate to the Modules tab:

Find Custom fields module, and click on the switch button to enable it.

You can add Custom fields directly from here by clicking Settings.

Please note that Custom fields feature is available in Ultimate plan only.
To add a new Custom field, click Add field button and define both fields in a pop-up window. Name your field, select a type and then click the green button Add:

You can choose Number or Text from the list:

Your Custom fields will appear in Projects tab.

Click on the name of a specific project or a task and the Edit panel will be opened on the right-hand side. Scroll down and you will be able to see your Custom fields there. Fill in the Value fields for your Custom fields and all changes will be automatically saved.

You can also access Custom fields settings from Projects tab by clicking the little arrow icon:
