How to aggregate SUM data using a criteria from a lookup field

How to aggregate SUM data using a criteria from a lookup field

I'm trying to figure out how to use the .sum feature when aggregating records, but only for records with criteria Account = Commerzbank

This formula works to add up all the Actual Amount field in the Add_Transactions form.
input.Balance = Add_Transaction.sum(Actual_Amount);

However, I only want to sum up records that have a critera Account = Commerzbank, but I continually get an error that Account is BIGINT and this doesn't work to search for a String. I can't seem to figure this out.

Thanks for the help