how to show javascript prompt box in c# code behind

Please check the correct answer[^] for the question Display a “Yes / No” alert box in C# code behind[^], where there are two solutions to do it in aspx and code behind page.

You can also go for this answer[^], where you have to use RegisterStartupScript[^] to build the confirm or prompt box.

[update] (as per the comments added by OP)

For your requirement (to show one prompt box to take input from User), you just need to write javascript codes for that using RegisterStartupScript.
You can find example of prompt box and codes at JavaScript Prompt Boxes[^]. You just need to write these codes inside RegisterStartupScript
Just try it out and let me know what problems you are facing.

[/update]

Thanks...