What is your external calendar management :
1/ Find your public address on Google agenda
Go to the My Calendars section on the left,
Move your mouse over the calendar you want to import to SamBoat and click on the 3 vertical dots.
Select Settings and sharing in the drop-down menu.
In "Permission and Access", press "Make publicly available".
Scroll down to the "Integrate Calendar" section and copy the public address in Ical format
Paste it in the field indicated on SamBoat.
2/ Find your public address on iCloud
In your iCloud account, go to the Calendar.
Click on the radar icon next to the calendar you want to import to SamBoat.
Check the box to make your calendar public.
Copy the link and paste it into the field on SamBoat