Greasemonkey/Tampermonkey @match for a page with parameters

According to the documentation for @match, it doesn't appear that query string parameters are something the Greasemonkey engine will match on:

https://developer.chrome.com/extensions/match_patterns.html

@match only works on the protocol/scheme, host, and pathname of a URL.

To trigger off the query parameters, you can either use @include or use @match and also test the URL yourself.
Note that the @match approach performs faster.

With @include, you can use a regex syntax. See, also Include and exclude rules.

In this case, use either:

...
// @include  /^https?://example\.com/page\.php*key1=value1*/
// ==/UserScript==

**Or:**

...
// @match *://example.com/page.php*
// ==/UserScript==

if (/\bkey1=value1\b/.test (location.search) ) {
    // DO YOUR STUFF HERE.
}