I asked a similar question earlier this week to their dev support around setting locations for sales order lines on new quotes/orders/invoices raised via the api and got the below response.
MYOB RESPONSE
As you've found, the Sales endpoints do not have a field to specify the location for stock on an invoice. The Multi-Location feature for AccountRight Premier users is an in product only feature as the API has no functionality for this.
There isn't any setting in AccountRight to choose a default location that is used on an invoice. When creating sales inside MYOB the location will always be blank and the user will need to select the location. If you POST a sale via the API, the Primary Location will be automatically selected. The user will have to manually change this if they want to use another location on the invoice.
Based on the above if you wanted a default location thats possible, however if on the same order you wanted to fulfill some lines from one location, and some other lines from another location then I dont think you can via the API (a user would probably need to amend the order/invoice via the U.I.).