Since GoodTask doesn't support file attachments yet, I am using links to OneDrive files.
The quickest and easiest way would be to use Share to Goodtask right from within OneDrive app. However, the GoodTask dialog that pops up only allows creating the task subject and picking the list name. For the link to work, it needs to go into Notes. It would be very helpful if "share to GT" popped up the full dialog, or had a button / link to access it.