During Purchase Order creation, you can use either the currency which is set for the selected Vendor, or the default QuickBooks Online currency.
If you want to publish requests in a currency that differs from your base (default) currency, you can add a Vendor with that foreign currency in QuickBooks Online.
This lets you record transactions in the currency you actually use to pay this Vendor.
You can add a foreign currency via the "I pay this Vendor with" drop-down menu in the Vendor's profile in QuickBooks Online, provided that the Multi-Currency option has been turned on in your QuickBooks Online profile.
Please note: Once you add a Vendor and record the transactions you have with them, you can't change the currency in their profile. The workaround here is to set the existing Vendor profile as inactive and create a new one with a new currency.