You can setup this up by following the below process to set the Any Currency setting on the supplier or customer account (please be aware that this setting cannot be disabled once switched on).
Select the following depending on whether you need to change a customer or supplier account
Customer: Debtors, Customers (ribbon menu), or Sales Ledger, Records, Customers (classic/module menu)
Supplier: Creditors, Suppliers (ribbon menu) or Purchase Ledger, Records, Suppliers (classic/module menu)
Locate the relevant record either by entering the relevant code, or via the search functions
Select Edit and tick the Any Currency box (you will be asked to confirm that you wish to switch this on as it cannot be changed once set)
Save the record
This will now allow you post transactions with multiple currencies. Note that switching this setting on also allows you to allocate in multiple currencies for the account in question.