The Textarea controls allow you to add a simple, multi-line text input.
Example
new \Kirki\Field\Textarea(
[
'settings' => 'textarea_setting',
'label' => esc_html__( 'Textarea Control', 'kirki' ),
'section' => 'section_id',
'default' => esc_html__( 'This is a default value', 'kirki' ),
]
);