There's no way you could find it without reading the manual.
The renderView marquee tool use Octane attributes defined in Camera:
as "use pick focus" is enabled the marquee tool is used to set the focus for depth of field,
but as "auto-focus" is enabled it overrides "use pick focus",
so yes by default the marquee tool does nothing (bad default and wording).
You have to disable "auto-focus" to set the focus using the marquee tool,
And you have to disable "use pick region" to make the marquee tool used for render region.
But be aware that the Octane "render region" as nothing to do with Maya render region,
this is more a "zone of interest", sampling is unlimited and this is used to get rid of noise on some region of a still image.
A true render region tool exists in Octane, it's called "film region" but it's not yet implemented in the Maya plugin.
