That does not work! I have a friend who had a play script stolen from her that she had registered that way, not only did they steal the script, but they secretly videotaped parts of her play and used them on a video for their company. My friend sued them and lost, but currently if fighting that ruling.
Always, always, always copyright your scripts-especially before sharing them with anyone claiming to be an industry representative of any kind. Posting scripts online, copyrighted or not, isn't the wisest of ideas either.