Yes, you can customize the first part of your E-junkie Marketplace Shop domain -- e.g. https://yourbrandname.e-junkie.com/. That would be done in your E-junkie Dashboard under Manage Cart & Shop > Shop Preferences > Shop URL.
Your original numbered Shop URL will always work and simply auto-redirect to whatever your current custom Shop URL happens to be. This help page explains more about how to set up your Marketplace Shop and product listings there:
The _source=admin URL parameter on a Shop URL is only used to add some help indications to the Shop page if a seller visits it from their own Dashboard before they've chosen to list any products in their Shop. It will not work for any other purpose.
However, you can implement Google Analytics and/or Facebook Pixel tracking in your E-junkie Shop and Product Card pages by simply pasting your tracking ID for those services in your account preferences -- see details here:
