runConfigurations

looks like it should extend one of Gradle's standard named collections, and the DSL adds special overrides for that, you should be able to do something like

create<...Application>("runIde") { ...

if it does

I see. Is this supposed to be obvious? Because the tools provided no assistance

I'm not sure. IntelliJ has provided me with completion for it before (it usually looks like the methods missing a type argument, and is slightly separate from the other similar methods) it's definitely something that should be documented somewhere, it might already be but I've never looked for it, I just explored in my IDE.