Currently, it is not possible to add more accounts under foreign VAT (reverse charge).
The easiest way to get around this is to create a cost center for this account in your accounting system and then synchronize your cost centers in Mynt.
This is how you sync your cost centers. May look different depending on the accounting system you use:
At the same time, you create a category in the chart of accounts that you name, for example, "Marketing outside the EU" or whatever the account is for.
Make sure to check "Reverse VAT" and don't forget to fill in the VAT rate, account number and category name as shown in the picture above. If you do these steps, the reverse charge will be booked correctly.