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