In a pipeline or Configuration-as-Code YAML, you can use this same comma-separated string form under the name hostSelectionCandidatesAsString, or instead give a native list of individual host names under hostSelectionCandidates - whichever is more convenient; both end up stored the same way.
A template or build step using this cloud can override this default with its own candidate list - including setting it to a single comma (,), which explicitly means "no restriction at this call site" and is treated differently from leaving it blank (which inherits this cloud-wide default).
Leave blank if there is no cloud-wide restriction to apply by default.