Add custom fields to a table to simulate a pivot table - Power Query

Hi,

I've used Power Query to merge 2 tables and to add multiple custom columns to simulate a pivot table.
However, I can't seem to add a "Month" filter field like in a normal pivot table.
Is there a way to do it?

I've tried to use the pivot table from Excel to do what I'm seeking, however I can't seem to produce what I'm looking for without
adding formulas next of the pivot table.

I'm also looking for something dynamic since the data will be updated every week.

Additional details:
I'm trying to add the following columns to the pivot table: (Nombre employés, Indisponibilté, Disponibilté, Taux disponibilté (%))

Thanks.