navigator.clipboard is undefined

you can write an all-in-one wrapper function.

• if in secure context (https) : use navigator clipboard api
• if not : use the 'out of viewport hidden text area' trick

// return a promise
function copyToClipboard(textToCopy) {
    // navigator clipboard api needs a secure context (https)
    if (navigator.clipboard && window.isSecureContext) {
        // navigator clipboard api method'
        return navigator.clipboard.writeText(textToCopy);
    } else {
        // text area method
        let textArea = document.createElement("textarea");
        textArea.value = textToCopy;
        // make the textarea out of viewport
        textArea.style.position = "fixed";
        textArea.style.left = "-999999px";
        textArea.style.top = "-999999px";
        document.body.appendChild(textArea);
        textArea.focus();
        textArea.select();
        return new Promise((res, rej) => {
            // here the magic happens
            document.execCommand('copy') ? res() : rej();
            textArea.remove();
        });
    }
}

use :

copyToClipboard("I'm going to the clipboard !")
    .then(() => console.log('text copied !'))
    .catch(() => console.log('error'));

ps : do not try it in a repl like jsfiddle/copeden/...

Try this:

if (typeof (navigator.clipboard) == 'undefined') {
    console.log('navigator.clipboard');
    var textArea = document.createElement("textarea");
    textArea.value = linkToGo;
    textArea.style.position = "fixed";  //avoid scrolling to bottom
    document.body.appendChild(textArea);
    textArea.focus();
    textArea.select();

    try {
        var successful = document.execCommand('copy');
        var msg = successful ? 'successful' : 'unsuccessful';
        toastr.info(msg);
    } catch (err) {
        toastr.warning('Was not possible to copy te text: ', err);
    }

    document.body.removeChild(textArea)
    return;
}
navigator.clipboard.writeText(linkToGo).then(function () {
    toastr.info(`successful!`);
}, function (err) {
    toastr.warning('unsuccessful!', err);
});

This requires a secure origin — either HTTPS or localhost (or disabled by running Chrome with a flag). Just like for ServiceWorker, this state is indicated by the presence or absence of the property on the navigator object.

https://developers.google.com/web/updates/2018/03/clipboardapi

This is noted in the spec with [SecureContext] on the interface: https://w3c.github.io/clipboard-apis/#dom-navigator-clipboard

You can check the state of window.isSecureContext to learn if that's the reason a feature is unavailable. Secure contexts | MDN

And yes, you should set up HSTS to make sure HTTP redirects to HTTPS.