how do you assign profile in a format plugin read sequence

I have a format plugin that will read various image formats.  I know what the source colorspace or profile  (i.e. ProPhoto RGB, sRGB, AdobeRGB, whatever) is.  I haven't figured out how to do the equivalent to Assign Profile from with my Read sequence in my Format Plugin.

Surely there is a way to do this.  ACR does it.

Any clues will be greatly appreciated!