Provided that your Intuit account features multi-currency support and you managed Currencies in the advanced settings of your QuickBooks Online Organisation, you can specify a different currency (other than your home currency) for a Vendor:
During Vendor creation, select the currency in which you will pay this Vendor:
During Purchase Order creation in ApprovalMax, the system will then use the currency specified for this Vendor as the base and convert the amount into your home currency using the exchange rate set in the QuickBooks Online settings for Currencies:
Please note: Once a currency has been selected for a Vendor, it will not be possible to change it again.