**This endpoint creates a new custom field definition.** Custom field definitions are created with the given `name` and `field_type`. Although field names are stored in a case-sensitive manner, all field names must be case-insensitively unique. This means you may create a field named `CamelCase` or `camelcase`, but not both. Additionally, a Custom Field name cannot collide with any Reserved Field names. You should save the returned `id` value in order to update or delete the field at a later date. You can have up to 120 custom fields. The custom field name should be created using only alphanumeric characters (A-Z and 0-9) and underscores (\_). Custom fields can only begin with letters A-Z or underscores (_). The field type can be date, text, or number fields. The field type is important for creating segments from your contact database. **Note: Creating a custom field that begins with a number will cause issues with sending in Marketing Campaigns.**