You could do something along these lines (it would be the same for one or more fields):

// Ensures the DOM (html) is loaded before trying to use the elements
window.addEventListener('DOMContentLoaded', function() {
  var cnInput  = document.getElementById("cn"),
      zipInput = document.getElementById("zip"),
      form     = document.getElementById("myForm");
      
  form.addEventListener('submit', getUrl); // On submit btn click, or pressing Enter
  
  function getUrl(e) {
    var cityname = cnInput.value,
        zipcode  = zipInput.value,
        url      = "https://sample.com/" + cityname + "/" + zipcode + ".html";
    
    alert(url);
    e.preventDefault(); // Prevent the form from redirecting?
  }
});

<form id="myForm">
  <label>Enter city name: <input type="text" size="12" id="cn"></label>
  <label>Enter zip code: <input type="text" size="12" id="zip"></label>
  <input type="submit">
</form>

First specify an action attribute for your form. This is where your form will be submitted. Then set your form’s method attribute to GET. Finally, add as many fields as you like (I am assuming you are after a GET query such as https:url.com?key1=val1&key2=val2...):

<form method="GET" action="https://sampleurl">
  Enter city name:
  <input type="text" size="12" id="cn">
  Enter zip code:
  <input type="text" pattern="[0-9]{5}"
  <input type="submit" ">
</form>

I have a bit of experience with HTML but am very new to JavaScript. In essence, I would like for a user input to be part of a URL. For example, we could have something simple such as:

<script>
  function get_cityname() {
  var cityname = document.getElementById("cn").value;
  alert(cityname);
  }
</script>

<form>
  Enter city name:
  <input type = "text" size = "12" id = "cn">
  <input type = "submit" onclick = "get_cityname();">
</form>

This will create a textbox where a user inputs their text (city name) and then click the ‘submit’ button next to it, and an alert should pop up based on the information they provided, just to make sure this works. However, this code only would seem to work (because of the ‘onclick’ command) to work for one user input. Therefore, I have 2 questions:

1. How could the above variable be included in a URL string? If it were something simple as:

URLstring = "https://sampleurl" + cityname + "moretext.html"

2. How could this be expanded if I want to include two or possibly even n number of inputs? For example, if I create more user prompt boxes and want to have the user also be able to input their zipcode, or state abbreviation, for example:

URLstring = "https://sampleurl" + cityname + "moretext" + zipcode + "moretext" + "stateabbreviation.html"