How to use parameters that go to a script in html content?

I am new to using Google script, but not completely new to programming. I have looked at some examples and they generate HTML output like this:

function doGet(e) {
  return HtmlService.createHtmloutputFromFile('form.html') 
}

I would like to pass a parameter to my script to use that in the output. I have gotten as far as that I can use this in the function:

var room= e.parameter.room 

So when I execute my script with url?room=test I do get the value for that parameter.

But how can I use that variable room that I create in the HTML output and other code?

I have been looking at createTemplateFromFile but I am not getting anywhere.

Hope someone can point me in the right direction of what constructs and command to look at.

You can directly get query parameters client side¹:

form.html

<script>
google.script.url.getLocation(function(location) {
  alert(location.parameter.room); //alerts "test" on loading  "url?room=test"
});
</script>

Alternatively, You can use scriplets to load html²:

code.gs

function doGet(e) {
  var temp = HtmlService.createTemplateFromFile('form');
  temp.room = e.parameter.room;
  return temp.evaluate();
}

form.html:

<script>
    alert('<?=room?>'); //Printing scriplets
</script>