Adding Pre-Determined Text/HTML To Post With Pulldown Selector

I will do my best to try and explain what I am looking for. I am sorry I couldn’t make the title more informative/direct, but I don’t know of a better way to sum this up.

What I want to accomplish is having my posts auto-insert text/html at the bottom upon publishing, ideally by having something like a pull down selector when creating/editing a post.

Depending on the option I select from the pull down when creating the post, I want the related text to be attached to the bottom of the post.

An example of a text would be something like “This post is displaying text from the first option in the pull down”, with it showing up below the content of the post.

I am sorry if I am vague and didn’t give enough information, but I really don’t know how to put this into better words. Any information on how to accomplish what I want is appreciated, an existing plugin that already does this would be ideal.

Thank you for taking a look and for any help you can offer.