To set @swisscows.ch as default sending address, open swisscows.email and do the following:
1. Open your @swisscows.email account and click on "Settings".
2. Under "Identities" click on the @swisscows.ch email address.
3. Now under "Settings" activate the slider "Set default".
4. Click "Save" to apply the desired settings.
And just like that, you can also change the settings back to @swisscows.email as the default sending address.