How to create Customer Text Field that references Project Field

I feel that what I am asking for should be easy but I am stumped, so your help would be great. 

I am creating a new field on a Task Form. The field is of type calculated text. In the field, I want to reference data from the Project, specifically a field on the project called 'reference'. 

I can see all the task fields but cannot see a way to reference back up to the project data the task is for. 

Thanks for any help you can give me with this one. 

Thanks,