K2 thinking can't be prefilled and is not available on the text completion endpoint. While these are not intended uses for the model they are still incredibly useful features that are enabled for most if not all other models hosted by Parasail.

Edit: I should note my assumption was wrong. This is the case for several other models such as both non-thinking K2 snapshots and GLM-4.6. It would be nice if text completion was available for them as well.