I want to send a post requesting for a story
First you go to the thread for the fandom your story is from. For instance, if it's a Twilight story, go here: You are not allowed to view links.
If it's for a different fandom, go t Fanfiction Alley and look for a thread for your fandom. If one does not exist, you can create one. But be sure to identify what fandom your story is in if you are creating a new thread.
I'm going to assume it's an existing thread and you've clicked on the thread to open it. Then you click the Reply button, found at the top or bottom of the page. You'll get a post box. Change your subject line to REQ Story Name by author name. Then fill in the box with your request information. Then click post. You can click on the Reply button from any page in the thread. Your post will automatically be posted on the last page of the thread. Then check back for a response.
If you're really asking about getting a story/author identified based on a plot line, you do the same thing, except put Story Search in your subject line.