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What are looking to do is not standard HTML coding anymore. A user enters information in the box, and you want to process it.
This now involves some type of scripting to read the entry and process the result.
There are many ways to do this, depending on the level of language skills you want to tackle and the operating system of the web host.
For a quick overview, I'd suggest looking at www.w3schools.com and search for "form processing".
A company I have dealt with in the past has just released a form processor software which looks pretty good. I like their other products, but have no need for this one.
There are probably also pre-built CGI programs to handle this which you could download and install.
It's the first step into a wider world.