Opera:

Go to 'Tools', then down to 'Preferences', click on the 'Advanced' tab,
then click on the 'Network' tab below:



Click the 'Proxy servers' button:



Please verify which case is yours:
- Case 1: Do you use a direct connection to the internet?
-> Nothing is selected (like in the screenshot above); go on with the Step 3 here.
- Case 2: Does your browser automatically detect the proxy settings?
-> Nothing is selected (like in the screenshot above); go on with the Step 3 here.
- Case 3: Do you use a proxy?



-> 'HTTPS:' is selected; please write down the URL or IP address from the 'HTTPS' field
    ('myproxy_https' in the example; please ensure to use the proxy settings from the 'HTTPS'
    field and not the 'HTTP' field) as well as the Port from the 'Port' field ('8080' in the example);
    go on with the Step 3 here.
- Case 4: Do you use a proxy script?



-> 'Use automatic proxy configuration' is selected; please write down the URL from this field
    ('http://myproxyscript/proxy.pac' in the example); go on with the Step 3 here.