add_action( 'personal_options_update', 'save_extra_profile_fields' );
add_action( 'edit_user_profile_update', 'save_extra_profile_fields' );

function save_extra_profile_fields( $user_id )
{
    update_user_meta( $user_id,'uidnumber', sanitize_text_field( $_POST['uidnumber'] ) );
}

<p class="form-row form-row-wide">
                <label for="uidnumber"><?php _e( 'UID-Nummer', 'woocommerce' ); ?></label>
                <input type="text" class="input-text" name="user_uidnumber" id="uidnumber" value="<?php echo esc_attr( $user->uidnumber); ?>" />
            </p>