There are hundreds of services that can help you achieve what you want here, but simplest case, you may not need any service to do this and can use what you have on your computer right now without really any experience with web technology:
If you're on Windows, open Notepad (Windows icon -> type 'notepad' or Accessories -> Notepad).
In notepad, copy the following snippet:
Code:
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>Document</title>
</head>
<body>
<!-- DELETE THIS ENTIRE LINE AND PASTE YOUR CODE HERE -->
</body>
</html>
Now, in the line beginning with <!-- ... -->, delete that whole line, start to finish, including the arrows, and paste in the code that your service is giving you.
Save this file (File -> Save / Save As), and save it to your desktop with the file name "test.html" or some other name that makes sense to you. Ensure that is has the .html extension, not .txt extension at the end.
Go to your desktop, double click it, and it should open in CHrome or whatever your default browser is.
(If it opened in another program like Notepad, the extension is wrong and you need to change it to .html. You can usually do this in NOtepad by opening the file, going to Save As, and in the file save dialog, change 'File type' to 'All' and then save it as a new file with .html at the end)
Caveats: SalesForce may require your website to be on a live server somewhere to test this functionality out... If it's just static HTML and static self-contained JavaScript it'll work where you can see your page in your browser, but if it requires some sort of two-way verification or some communication back to salesforce, not only will the form not function, it might not even display/render. It depends on the code you're adding in here and what the desired result is.
If this is the case, you can use a quick website testing tool like Glitch.com. GO to New Project -> Website. It'll open default static site, with a file editor. In the left side menu, click on the "index.html" file, and it'll look mostly like my snippet above. Paste your code anywhere in the body or if CC has instructions to paste it elsewhere, do that. THen click the 'Show' button in GLitch and it'll launch your page in another tab. Caveat on this: ConstantCOntact might add an API key or other private information in the code snippet they give you, they probably don't for front-end (e.g., html and JS pasted into a webpage), but they might. Just... be weary of that.
Caveat two: In my code example above... you really don't need 98% of that to try this, but ... it's the snippet that my code editor of choice provides when you tabcomplete the word
doc... so it was easier for me to just copy all of it.
Caveat Three: If you're on a Mac you can do the same thing with Textpad or whatever the default text editor is.
