Bringing in the first string in a list, based on matching values from two columns

[jabbajuice08]

This is how two of my columns are set up. The ID# is unique and any MMM named ID #'s have nothing in the Link ID # because they are the 'parent' or 'starting' task. Any tasks marked with the same string in the Link ID # column, are associated under that ID # (I have one data set and a duplicate of the same dataset for drilling through).

My Matrix is set up like this, but on PowerBI: 

You can expand 'MMM tasks' and look at all the unique MMM ID #'s/parent tasks. When you click on any of the ID #'s, for example 'MMM3', you would get a list of tasks associated with that string (These data points are found in the duplicated data set that match the original dataset:

I am trying to obtain this last column on my matrix: 

I need to look at that same list of tasks in my measure/calculated column that show up when you click on each MMM task. It then brings in the first task ID # in that list. Since most dates are the same, I just need to bring in the first task that shows up when you drill-through to the data. I've tried a few ways of doing it (Lookup value, Firstnonblank), but I seem to be doing something wrong. How do I go about showing that in my matrix properly? 

[lbendlin]

It then brings in the first task ID # in that list. 

Power BI has no concept of that. You need to provide guidance on what "first" means. is it done by sorting the ID # field alphabetically?

On an unrelated note - are you aware of the PATH functions?

[jabbajuice08]

I have it sorted by the finish date of the task, but I'm not sure how powerbi sorts it after that if there are multiple tasks with the same finish date

[lbendlin]

That is exactly the type of ambiguity you want to avoid.  Best to bring your own index column.