At the moment any utm_key value supplied to donatewiki is ignored and overwritten by the extra tracking data in DonationForm.js. It should be fairly easy to change this so the extra tracking data is appended to any utm_key which is supplied.

This is needed to make T219793#6265572 (tracking mobile sidebar donations via utm_key) work, but could also have other applications e.g. email links adding data to utm_key.