How to set breakpoints in vs code in a scala program

I want to debug my scala app with vscode.

My gradle run task looks as simple:

task run(type: JavaExec, dependsOn: build) {
    debug true
    main = mainClassFile
    classpath sourceSets.main.runtimeClasspath
    classpath configurations.runtime
}

The launch entry:

{
    "type": "java",
    "name": "Debug (Attach)",
    "request": "attach",
    "hostName": "localhost",
    "port": 5005
 }

Now when I launch the debugger, it waits for the vscode client. But I am not able to set any breakpoints in vscode in order to stop the debugger. For now it runs until the end, after vscode connected.

I am using the Scala and Scala Language Server.

Does someone know how to add breakpoints?