You can define your Default Site from Community Site Settings in the Lightning Lens App.
Changing your Default Site affects the following areas:
All Site URLs
If there is no Default Site set in Lightning Lens, a “URL No Longer Exists“ error generates when accessing Financial Documents. Define a Default Site here.
Enable Apex Settings:
The Default Site will revert to a blank/null state if Apex Settings are not enabled to deploy metadata from non-certified package versions.
Navigate to Setup > Apex Settings and ensure Deploy Metadata from Non-Certified Package Versions via Apex is enabled. Only attempt the below steps after this setting is enabled. It is important this setting is enabled for your Community Site to run correctly.
Define Default Site:
Navigate to the Fonteva Framework tab and Select Apps from the left-side menu.
Click the Lightning Lens app.
Scroll Down to the Settings Page(s) section and Click Community Site Settings. The Community Site Settings page will open.
Provide your desired site in the Default Community Site field. Results will generate as you type. You may Select a site from the results.