Azure App Service is a service used to create and deploy scalable, mission-critical web apps.
In Azure portal: select the app's Configuration page, then select General settings. In the Startup Command field, place either the full text of your startup command or the name of your startup command file. Then select Save to apply the changes.
stack: Java SE
Expected value: the command used to start your JAR app (e.g. java -jar /home/site/wwwroot/app.jar --server.port=80)
Kindly see, this document for more details.
Also, refer the sample/step-step process: Quickstart: Create a Java app on Azure App Service