Button on click to check a particular checkbox ID with a Tampermonkey script

I would like some help with a Tampermonkey script.

I have a script that will add a button to the page and on click, it will add some text to a text area. That part is working fine. See below.

Now, I need that on click of this button, before it adds the text, I need it to check a checkbox ID="1234"

Please assist.

---

(function() {
 window.addEventListener("load", () => {
 addButton("Add Markdown");
 });

function addButton(text, onclick, cssObj) {
cssObj = cssObj || {
  position: "fixed",
  top: "15%",
  right: "4%",
  "z-index": 3,
  fontWeight: "600",
  fontSize: "14px",
  backgroundColor: "#00cccc",
  color: "white",
  border: "none",
  padding: "10px 20px"
};

let button = document.createElement("button"),
  btnStyle = button.style;
document.body.appendChild(button);
button.innerHTML = text;
// Setting function for button when it is clicked.
button.onclick = selectReadFn;
Object.keys(cssObj).forEach(key => (btnStyle[key] = cssObj[key]));
return button;
}

function selectReadFn() {
var txt = document.getElementById("markdown-editor").value += '\n---\n```markdown text here.\n```\n\n'

}
})();

I couldn't find a script that will check a checkbox with the click of another button