Hi there @goshdark138! Thank you for your interest in ownCloud!
You can change the theme and build your own custom OwnCloud Desktop client by modifying src/libsync/theme.cpp. In particular, to do what you are asking, you can modify Theme::overrideServerUrl() function in order to hardcode which URL you want as predefined:
Just return QString("http://your.custom.owncloud.URL"); there and you got it!
If you need further branding options you could try ownBrander (part of the OC Enterprise Ed. perks) to do so, it's way more comprehensive than modifying the code and abstracts all that work for you 