### If you have a subscription
$ sudo su
# URL=https://serial:<License key>@download.kopano.io/supported/webapp:/final/Ubuntu_18.04/
# echo "deb $URL /">> /etc/apt/sources.list.d/kopano-webapp.list
# curl $URL/Release.key | apt-key add -
# apt install kopano-webapp
# exit
### If you have the Community Packages
$ wget https://download.kopano.io/community/webapp%3A/webapp-4.2.2860%2B1697.1.12a4e21-Ubuntu_18.04-all.tar.gz
$ tar xfz webapp-4.2.2860+1697.1.12a4e21-Ubuntu_18.04-all.tar.gz
$ cd webapp-4.2.2860+1697.1.12a4e21-Ubuntu_18.04-all
$ sudo dpkg -i *.deb
### If dependencies cannot be resolved:
$ sudo apt install -f