In this example we will redirect http://example.com to https://example.com/mywebsite
Open /etc/apache2/sites-available and find:
DocumentRoot /var/www/html
We will comment this out like so:
#DocumentRoot /var/www/html
Under that add line add:
RedirectPermanenet / https://example.com/mywebsite
Save the file and restart Apache:
systemctl restart apache2
If you want to redirect https then do the same thing in /etc/apache2/sites-available/default-ssl.conf