Unfortunately it seems that Safari hijacks all google.com/search?q= urls and strips all (or at least utm) query params, except q, when google is set to the default search engine!

Even pasting in https://google.com/search?q=apple+sauce&utm=14 to the url bar doesn't work. The only way I was able to get this to work was set DuckDuckGo (or anything non-google) as the default search engine, then use the extension "Smart Keyword Search" to setup a shortcut. I have !g <query> for normal google search and !gw <query> setup for google "web" search.