Currency Conversion Power Bi

You could do it as a measure like

USD Amount Measure =
SUMX (
    'Transactions',
    VAR CurrencyRate =
        LOOKUPVALUE (
            'Exch rate'[rate],
            'Exch Rate'[Date],
                DATE ( YEAR ( 'Transactions'[Transaction date] ), MONTH ( 'Transactions'[Transaction date] ), 1 ),
            'Exch rate'[Currency], 'Transactions'[Currency]
        )
    RETURN
        'Transactions'[Amount] * CurrencyRate
)

but I think performance would likely be better having a calculated column and then building measures based on that. If it was a column then it would be calculated once, during data refresh, and then you could have measures to perform sum, average etc on the calculated column without having to pay the cost of all the lookups.

Best advice is to try both ways and use DAX Studio to compare performance