First enable the proxy modules in Apache:

sudo a2enmod proxy
sudo a2enmod proxy_http
sudo /etc/init.d/apache2 restart

Then you will have to edit /etc/apache2/mods-available/proxy.conf file. But before that backup your proxy.conf file:

cd /etc/apache2/mods-available
sudo mv proxy.conf proxy.conf.default

Now create a new file called proxy.conf using your favorite text editor and add the following lines to it:

<ifmodule mod_proxy.c>
  #turning ProxyRequests on and allowing proxying from all may allow
  #spammers to use your proxy to send email.

  ProxyRequests Off
  <proxy *>
    AddDefaultCharset off
    Order Allow,Deny
    Allow from all
  </proxy>

  ProxyPass /transmission http://localhost:9091/transmission
  ProxyPassReverse /transmission http://localhost:9091/transmission
  # Line below is optional
  Redirect permanent /transmission https://myserver.com/transmission/web/

  # Enable/disable the handling of HTTP/1.1 "Via:" headers.
  # ("Full" adds the server version; "Block" removes all outgoing Via: headers)
  # Set to one of: Off | On | Full | Block

  ProxyVia On
</ifmodule>

Source: Transmission Apache proxy setup