In this example you can create a run.bat with the following syntax and use pass the java parameters you prefer:
set PATH=%PATH%;%JAVA_HOME%/bin
java -Xmx1024m -jar helloworld.jar Zip all run.bat with your jar and upload it to azure webjob...
Web Job Reference: https://docs.microsoft.com/en-us/azure/app-service/webjobs-create