Unable to pass multi select parameter to a parameterized build

I cannot find any way to pass parameter value to Extended Choice Parameter (Multi Select).

JobWithDetails job = jenkins.getJob("Build-Multiple");
Map params = new HashMap<>();
params.put("packages", "test1,test2,test3");
job.build(params);

Start from the JobWithDetails.build(Map) call shown in the issue and trace how the packages value is handled for an Extended Choice Parameter (Multi Select). Reproduce the test1,test2,test3 case, then establish the supported parameter representation and verify it with a regression test for a parameterized build.

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