[racket] Scheme within HTML (Web-Server)

Hi,
after i tried to create a html page with Racket embedded and got the answer
from you that I schould use WebServer library, now i can't get how it realy
works and need help!

I want to create a simple website as a online course for Scheme Language
for beginners! my goal is to explane the general functions and operations
and so on in scheme but i want  to let the reader be able to try some code
and by clicking a button see the result!
ex. he makes input: (+ 1 4) and get result 5 withen my page!
How can i write a html code and let Scheme code be exicuted from the html?
could you explane me with my simple (+ 1 4), please?
I'm using DrRacket!

thank you..
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