Hi @lasse 

There is a hidden column called Related Content that will auto generate a link back to the original workflow item in a task list, It doesn't show the ID but does directly show the link and when referenced in a workflow it does return an ID, However the article that I believe that details how to turn this on has been archived, I am trying to re-instate it to see if it has the solution.

Looking around online I cannot find any definitive answers other than to use JSLINK to create a workaround but in my solution that I have working for this it isn't using JSLINK.

Apologies I don't have the solution right now but I didn't want to just leave your question unanswered, I will try to find out how I did this for you. 

In my own production environment I need to populate the Task List Items with information regarding their Related List Item, and I *think* that the approach I'm using would solve your issue.

There is a Item Property on Task List Items called Workflow Item ID:

It's populated with the ID of the Item that is related to the Task (assuming that your Workflow was running on a list item!), but there is a terrible catch. It's typically not populated until a bit AFTER the task has been created!

The only way that I've found to reliably get around this downside is to use a standard Loop Action that will continue to loop while the Workflow Item ID is less than 1:

This way, as soon as the value becomes available, I break out of the loop and it can be used in any actions beyond that point (Queries, Item Updates, etc.).

Let me know if that gets you the info you're looking for.