Custom Fields allow you to create contact-specific fields that store additional information on a contact profile. Use them to extend the standard fields and capture whatever details you need.
Navigate to Settings (bottom left).
Select Custom Fields from the settings menu on the left.
Click All Fields (top right).

Click Add Field.
In the pop-up, choose the type of custom field you want to create.

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Note: Once you create a custom field, you cannot change the field type. To change the type, delete the field and create a new one.
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Custom field types