Opening Balance Non-Std Calendar

How would I create the opening balance for a fiscal period that doesn’t always start on the 1st of the month.
I’m trying to create a monthly cashflow.
The OPENINGBALANCE or STARTOFMONTH function doesn’t work for me

If you could post an example I think that would be helpful here.

Intuitively you would just need to find the the first number in a column that represents the beginning of that fiscal period, which can be done.

If you can post an example of scenario in a pbix file which also contains how you want to showcase the information (ie. what context will this formula be placed in)