Add text or code at the top of Multiple Posts

Hello,

I want to have kind of announcement on some of my posts, but I don’t want to edit each post one by one.

Do any one know if there’s any plugin or quick way, that I can input a block of custom text/code, then choose multiple posts to have the custom text/code appear at the top of multiple posts? Or I must edit it in code level?

Appreciate if anyone can give some advice. Many thanks!