I remember a time when there were whole libraries dedicated to clipboard management.

Turns out in browsers released after ~2020 you can do it with one line:

navigator.clipboard.writeText(text).then(console.log).catch(console.error);

Some caveats:

• this only works in a secure context (which mostly means either “a site served over HTTPS” or localhost).
• the user needs to have interacted with the page before you can actually call the method.
• this was only added to certain browsers in 2020, so it's new enough that it might cause problems if your constituents have older devices or don't update often.

Usually when I do this I'm implementing something like a “copy to clipboard” button:

<span id='copy-target'>This text is going into a clipboard</span>
<button onclick='copyToClipboard("copy-target")'>Copy</button>
<script>
    const copyToClipboard = (target) => {
        const element = document.getElementById(target);
        const value = element?.innerText
        if (value) {
            navigator.clipboard.writeText(value).then(console.log).catch(console.error);
        }
    }
</script>