I'm wanting to make a search function in a limited way.

The following code sends the input text to a page where the script processes it, no problem

Code:
<form action="search.php">
  <input type="text" name="site_search" />
  <input type="submit" value="Search" />
</form>
I would like to have text automatically added to the input.

Example: I have a site I want to be only about things that are red.

Someone inputs "table" and the text "red+table" is sent to the search page.