Yes, with an "&". The POST string should look pretty much identical to a querystring, without the leading "?". Note that, typically, it should also be URL encoded like a querystring, as well.
urlTransfer = CreateObject("roUrlTransfer")
urlTransfer.SetUrl("https://api.blah.com/ath/link")
urlTransfer.AddHeader("Content-Type", "application/x-www-form-urlencoded")
result = urlTransfer.PostFromString("client_id=lOnGStringOfCharacters&client_secret=reallyreallylongstringofcharacters&grant_type=password&username=blabla%40blababla.com&password=s0mth1ng&redirect_uri=http%3a%2f%2fmyblah.com%2ffail")
My Channels: http://roku.permanence.com - Twitter: @TheEndlessDev
Instant Watch Browser (NetflixIWB), Aquarium Screensaver (AQUARIUM), Clever Clocks Screensaver (CLEVERCLOCKS), iTunes Podcasts (ITPC), My Channels (MYCHANNELS)