edit_form_after_editor.php

<?php
// エディタの下にテキストエリアを追加
add_action( 'edit_form_after_editor', 'nskw_postMetaAfterEditor' );
function nskw_postMetaAfterEditor() {
  global $post;
	$metaKey = 'afterEditor';
	if ( empty ( $post ) || 'post' !== get_post_type( $GLOBALS['post'] ) ) {
		return;
	}
	if ( ! $content = get_post_meta( $post->ID, $metaKey, TRUE ) ) {
		$content = '';
	}
	printf(
		'<p><label for="%1$s_id">お問い合わせ先</p>
		<textarea name="%1$s" id="%1$s_id" cols="60" rows="5">%2$s</textarea>',
		$metaKey,
		esc_attr( $content )
	);
}
// テキストエリアの内容を保存
add_action( 'save_post', 'nskw_savePostMeta2' );
function nskw_savePostMeta2( $post_id )
{
	if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) {
		return;
	}
	if ( ! current_user_can( 'edit_post', $post_id ) ) {
		return;
	}
    $metaKey = 'afterEditor';
    if ( isset ( $_POST[ $metaKey ] ) ) {
    	return update_post_meta( $post_id, $metaKey, $_POST[ $metaKey ] );
    }
    delete_post_meta( $post_id, $metaKey );
}