SQL: Select wordpress posts with given text string and add a custom field to them?

Here you go (you’re really close):

INSERT INTO wp_postmeta (post_id, meta_key, meta_value)
SELECT ID AS post_id,
'customer_reviews' AS meta_key, 
'On_or_delete' AS meta_value
FROM wp_posts WHERE wp_posts.post_content LIKE '%Customer Reviews%'
   AND wp_posts.post_type="post";

`

Edit:
You could also just change IS IN to IN. I rewrote the from part of the query since a subquery was unnecessary though.