using createAndRetrieve how to assign a value to a 'Reference to an Object'

For example, I am creating tasks, but I also want to link them to a project.

It looks like that should be done at the task with the api field of ParentProject.

But what value is it expecting?

I have tried to put in the parent' projects values for:

External ID,

ID,

Name

But using the value for any of those does nothing.

What is the proper way to create a task and assign it to an existing project?