How to set up cross domain requests in Laravel + JS?

Question

CORS

There is a widget that is embedded on the site foo.ru and makes a request to its server on bar.com :"

function getDataWidget(config) {
    let xhr = new XMLHttpRequest()
    xhr.open('GET', `https://bar.com/widget/${config.key}`)
    xhr.responseType = 'json'

    xhr.onreadystatechange = function() { 
        if (xhr.readyState !== 4) return; 
        if (xhr.status === 200)
        {
            return initWidget(config, xhr.response)
        }
        else
        {
            console.log('HTTP error', xhr.status, xhr.statusText);
        }
    };

    xhr.send()
}

to display data from this server to the foo.ru site .
I get the usual error:

Access to XMLHttpRequest at ' https://bar.com/widget/b1Mz3t9XVXefDGyk ' from origin ' https://foo.ru ' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource.

HTTP error 0

On other sites they write incomprehensibly. Can you please explain how to make the widget able to receive data from its server for the site it connects to?