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, e.g. hostSelectionCandidates: ['esx01.example.com', 'esx02.example.com'] - whichever is more convenient. Both end up stored the same way; use only one of the two per template/step.
Not every host visible in a cluster is necessarily usable by the vCenter account running this connection - a vCenter admin may restrict provisioning permission to a subset of hosts. This field is the only mechanism this plugin offers to keep automatic selection within permitted hosts.
This is not verified live against actual vCenter permissions: an incorrect entry, or a host the account cannot actually write to, only surfaces as a vCenter-side error when a clone is attempted, not as a validation error here.
Leaving this blank inherits the cloud's own default candidate list (configured on the vSphere Cloud itself), if any; if the cloud has no default either, every (usable) host in the configured cluster is a candidate. To explicitly override the cloud's default to "no restriction" for this call site specifically - rather than inheriting whatever the cloud has configured - enter a single comma (,): this is not blank, so it is treated as a deliberate override, but it still parses to zero host names.