Need help in integrating third-party systems in Magnolia CMS. Preferably, a commenting or a forum system since Magnolia doesn't support these modules anymore. Some third-party system that Magnolia suggested are: Disqus, IntenseDebate and Discourse.
source: https://documentation.magnolia-cms.com/display/DOCS56/Commenting+module
What exactly do you need? If you need to integrate with a third party service, it should pretty easily doable via REST and almost all the services provide REST APIs these days.
That being said those modules are no longer supported but if you want to use them you still can (just add them to the bundle) that being said perhaps you need to migrate them to 5.6 then please have a look at: https://documentation.magnolia-cms.com/display/DOCS56/Upgrading+to+Magnolia+5.6.x#UpgradingtoMagnolia5.6.x-Vaadin8andcustommodules
HTH,
Cheers,