Pop up on clicking Related Action button

On the related action you have to call a process model with a start form enabled. This start form just shows the warning message with the possibility to cancel the request. Exactly the way as ManuelHTG suggested.

=a!formLayout(
  label: if(
    isnull(ri!record.title),
    "Delete?",
    "Delete " & ri!record.title & "?"
  ),
  instructions: "",
  contents: {
    a!paragraphField(
      labelPosition: "COLLAPSED",
      readOnly: true,
      value: "Submitting this form will delete your record. The data will no longer exist or be visible.

After deletion, it is recommended to return to the records list, as this record will become unavailable."
    )
  },
  buttons: a!buttonLayout(
    primaryButtons: {
      a!buttonWidgetSubmit(
        label: "Delete",
        style: "PRIMARY",
        confirmMessage: "Are you sure you want to delete this record? This action cannot be undone."
      )
    },
    secondaryButtons: {
      a!buttonWidgetSubmit(
        label: "Cancel",
        style: "NORMAL",
        value: true,
        saveInto: ri!cancel,
        skipValidation: true
      )
    }
  )
)

On the related action you have to call a process model with a start form enabled. This start form just shows the warning message with the possibility to cancel the request. Exactly the way as ManuelHTG suggested.

=a!formLayout(
  label: if(
    isnull(ri!record.title),
    "Delete?",
    "Delete " & ri!record.title & "?"
  ),
  instructions: "",
  contents: {
    a!paragraphField(
      labelPosition: "COLLAPSED",
      readOnly: true,
      value: "Submitting this form will delete your record. The data will no longer exist or be visible.

After deletion, it is recommended to return to the records list, as this record will become unavailable."
    )
  },
  buttons: a!buttonLayout(
    primaryButtons: {
      a!buttonWidgetSubmit(
        label: "Delete",
        style: "PRIMARY",
        confirmMessage: "Are you sure you want to delete this record? This action cannot be undone."
      )
    },
    secondaryButtons: {
      a!buttonWidgetSubmit(
        label: "Cancel",
        style: "NORMAL",
        value: true,
        saveInto: ri!cancel,
        skipValidation: true
      )
    }
  )
)

thanks juergeng393, it worked...