How to configure the Heroku procfile to find the application path that is not at the root of the project?

Question

I have a springboot application in the git repository with the following configuration:

/api 
.gitignore 
Procfile 
README.md

However when I try to do Deoloy on Heroku I get the following error:

!     No default language could be detected for this app.
        HINT: This occurs when Heroku cannot detect the buildpack to use for this application 
        automatically.
        See https://devcenter.heroku.com/articles/buildpacks
!     Push failed

The configuration I have in the file Procfile this way:

web: java $JAVA_OPTS -jar api/target/MoneyCtrl-0.0.1.jar --server.port=$PORT

Does anyone know how to set the path for Heroku to find the application inside the /api folder ?