Select your previously created Project in the Organize Library (1). As you didn't add any Tasks yet the Task List is empty (2).
Click "Add Task" (1) or press Command + on your keyboard to add a new Task. The new task is automatically assigned to the currently selected project and the project name is displayed in the upper right corner of the task (2)
Organize:Pro has a setting to automatically activate the in-line editor, whereas Organize:Me automatically activates the Task Inspector.
You can easily move a Task to a new Project by Drag and Drop.
Go the the Inbox (1) and drag "Assign Review Dates" (2) to the left and drop it on "My first Project" (3). Your task is immediately moved to "My first Project". Select the Project to verify it.