I don't believe it is currently possible to use the query string, only the path parameter when using a Microflow URL. This means using your example you could get the my-utm value, but not the utm_source value.
If it's just the my-utm value you need, give your microflow a parameter and use this in the URL.
https://docs.mendix.com/refguide/microflow/#url
If you need the query string values as well, then it is probably best to stick to using the Deeplink module.
Good luck!