Hi Andres,
I guess the issue is that getElementsByClassName is returning a list of results (note the s after Element). So you should update it to update all occurrences (or the first or something different). Also, insertAdjacentElement expects an Element, not a string.
Try something like this:
// BEGIN USER CODE
const NewText = document.getElementsByClassName("mx-text mx-name-text3");
Array.from(NewText).forEach(element => {
element.innerHTML = "Boop <b>boop</b>!";
const newParagraph = document.createElement("p");
newParagraph.innerHTML = "boop boop underneath";
element.insertAdjacentElement("afterend", newParagraph);
});
// END USER CODE
Good luck!
Johan