After you turn on multicurrency (which you can do from a web browser by following the steps here), you can choose who pays you with a foreign currency.
- From the Menu ☰, select Customers.
- Fill out the customer's details.
- Under Customer pays me with, choose the currency.
- Select Done.
Read more about multicurrency here.
Note: Use the same steps to assign a foreign currency to a customer by selecting I Pay This Supplier With and then select the currency.