Hi,
The idea is to set up an extra virtual host for webmail site in your Apache configuration. You might create a subdomain, e.g. webmail.example.com, pointing at droplet’s IP, and configure Apache virtual host for that subdomain, entering document root directory (site directory) different to the directory of your main domain (example.com). So, main part of webmail virtual host configuration would look like:

<VirtualHost *:80>
    ServerName webmail.example.com
    DocumentRoot /var/www/webmail
    ...
</VirtualHost>

Here is the tutorial on that subject (especially step 5):
https://www.digitalocean.com/community/tutorials/how-to-install-the-apache-web-server-on-ubuntu-18-04#step-5-%E2%80%94-setting-up-virtual-hosts-(recommended)