You can add any type of live chat widget to the uSkinned Site Builder. We even use one on our main uSkinned.net website.
We use Tawk (https://www.tawk.to/) and this is actually free to use so you should have a look at it, you can even download an app to your phone so you can Live Chat via your phone to customers. Although there are plenty of other Live Chat widgets out there (Elfsight Apps, Live Chat, Hub Spot, Intercom) a quick search on Google will help you find the one that suits your needs.
Sign up for any Live Chat service like Tawk.
Copy the chat widget code provided.
Login to your Umbraco CMS > In the tree menu, go to 'Global' > From the grid, go to 'Settings'.
Go to the 'Scripts' tab.
Paste the code you copied earlier into one of the fields provided. Typically the live chat widget provider will let you know where on the page the code should be copied to.
Save and Publish. Your live chat widget will now be present on all pages within your website.
If you would prefer to only show your Live Chat widget on certain pages then instead of pasting the code into Global Settings > Scripts, you can go to any page in the CMS and find 'Scripts' tab. You should then paste the code into the field on the page you would like to see the widget appear.