Add meta data field to a user.

Post meta data is called “Custom Fields” on the Administration Screens.

Signature

add_user_meta( $user_id, $meta_key, $meta_value, $unique = false )
user_id
 (int) User ID.
meta_key
 (string) Metadata name.
meta_value
 (mixed) Metadata value.
unique
 (boolean) Optional, default is false. Whether the same key should not be added.
Default: false

Return

(int|boolean) Meta ID on success, false on failure.

Source

function add_user_meta($user_id, $meta_key, $meta_value, $unique = false) {
	return add_metadata('user', $user_id, $meta_key, $meta_value, $unique);
}
WP Trac GitHub

Link here