Selecting the Use Project Id radio button enables the Browse... button. Click the Browse... button to select an existing Project ID or create a new Project ID:
This dialog displays a list of existing Project IDs and allows you to create a new Project ID. The user has to be logged in to use this feature; otherwise the list will be blank. Users can sign in by clicking the link in the dialog.
If you'd like to create a new Project ID, click the Create Project Id button, which opens the Create Project Id dialog:
You can then type in your new Project ID and click Create. If the Project ID is valid and not taken, then it is created, and you'll be taken back to the Select Project Id dialog, which is now updated with the newly-created Project ID.