Bitstamp API USD purchase of BTC at marketvalue

[Mpproduct]

Taking the first steps with Bitstamp API, we quickly got stuck trying to purchase some BTC based on specific amount of USD we want to invest.

I see there is an option to execute "market order"  / "limit order" but what I am looking for is something like the simple "Instant order" functionality on Bitstamp site:
I want to buy in exact USD amount (as long as it's over 5 USD per terms and conditions) and not ask for specific BTC amount