If I understand you correctly you want to center an already created button, it could work something like this:

<!--In only HTML-->

<button style="position: absolute; left: 50%; transform: translateX(-50%);">Click Me</button>

Another way to do it is to set an id to the button and then center it in a separate CSS file or in the <style> tag like so:

<!--HTML-->

<button id="myButton">Click Me</button>

/*  

  CSS 

  Remember to use a # before the name because it's an id you're trying to reach

*/

#myButton {

  position: absolute;

  left: 50%;

  transform: translateX(-50%);

}

Or you could use JavaScript with the same HTML base

<!--HTML-->

<button id="myButton">Click Me</button>

And then in a separate JavaScript file or in the <script> tag

// Javascript

let myButton = document.querySelector("#myButton");  myButton.style.position = "absolute";

myButton.style.left = "50%";

myButton.style.transform = "translateX(-50%)";