The best approach for this problem is as follows:

Click Group By from the Transform Banner:

Group By: pick the columns that you need to keep their name, pick the "Median" operation (important!) and any other measures that you need (I, for example, needed "Count"):

Now that I have the "Median", AKA List.Median, I can easily change it to List.Percentile ([Value], 0.9). Right Click "Advanced Editor" and change the code accordingly:

And change the code in the "Grouped Rows" section from:

{"Per90th", each List.Median([Value]), type nullable number}}

{"Per90th", each List.Percentile([Value],0.9), type nullable number}}