How do I apply the UserTrust Certificate on an unsupported version of macOS?

NOTE: If the website that you are trying to download the USERTrust CA using Chrome is not working, you can choose to ignore the warning and proceed to the website.

Click on the Advanced link in the Error Message.
Next, click on Proceed to [URL] (unsafe) option.

Chrome browser will take you to the website that you were trying to access and you should be able to browse the website, without any problem.

NOTE: If the website that you are trying to download the USERTrust CA using Chrome is not working, you can choose to ignore the warning and proceed to the website.

Click on the Advanced link in the Error Message.
Next, click on Proceed to [URL] (unsafe) option.

Chrome browser will take you to the website that you were trying to access and you should be able to browse the website, without any problem.

Download the USERTrust CA.
Go to Keychain Access on your Mac (you can do this by pressing Command + Spacebar to bring up Spotlight Search and then just type in Keychain Access).
Drag and drop the USERTrust CA in Certificates.
Right-click on the USERTrust CA, and select Info.
Under Trust, click Always Accept.