Add a Textarea field to the WordPress Customizer.

WP Customizer - Textarea

$wp_customize->add_setting( 'themeslug_textarea_setting_id', array(
  'capability' => 'edit_theme_options',
  'default' => 'Lorem Ipsum Dolor Sit amet',
  'sanitize_callback' => 'sanitize_textarea_field',
) );

$wp_customize->add_control( 'themeslug_textarea_setting_id', array(
  'type' => 'textarea',
  'section' => 'custom_section', // // Add a default or your own section
  'label' => __( 'Custom Text Area' ),
  'description' => __( 'This is a custom textarea.' ),
) );