To add custom fields:

From the main menu, click on and then Company Settings. From the left, under Company section, click Custom Fields.
On the Manage Custom Fields pop-up, click on Add.
Select an appropriate type of custom field you want to create.
In the Add Custom Field, type name in the Name* box.
In the Formula Key* box, type the formula key.

Formula Key should be unique, must start with a letter (capital or lower case) and can contain underscores and numbers.
It should not contain special characters like #,!, etc and should not start with a number. This key is used when referencing the custom field in API.
Under the Attached to tab, select the appropriate entity.
Fill in other details and click Save.

Some important points to note:

A custom field when attached to an entity will appear on its details page.
For example a custom field attached to a User will appear on its details page.
The value of the custom field can be viewed from the details page. You will be able to enter/edit its value its from the 'Custom Fields' tab by editing that entity.
Only a custom field of type Multi-select drop-down can be attached to a Task.

To edit custom fields:

From the main menu, click on and then Company Settings. From the left, under Company section, click Custom Fields.
Point to a custom field, and click on the Pencil icon to edit the field.
In the new popup, edit the required details under the different tabs (Basic and Attached To) and click Save.
The custom field is saved with the new changes.

To delete custom fields:

From the main menu, click on and then Company Settings. From the left, under Company section, click Custom Fields.
Point to a custom field and click Delete icon.
Confirm the deletion by clicking on Yes. The custom field is deleted.

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