Prerequisites

Java
Windows environment

Procedure

Download the certificate. (It's easiest to open it in a browser by specifying the host and port number, then export it from "Certificate Details" and save it to your local disk.)
Run Command Prompt as 'Administrator'. (Administrator privileges are necessary to add to the keystore.)
Execute the following command:

"c:\Program Files\Java\jre1.8.0_241\bin\keytool.exe" -import -trustcacerts -file c:\work\yourhost.crt -keystore "c:\Program Files\Java\jre1.8.0_241\lib\security\cacerts"