For a Composite Task you need to define executors and set AssignedTo field. This field has a specific type InArgument<Collection<string>>. So you need to build a collection of strings and specify it to Composite Task activity. Let's take a look.
You can see the AssignedTo parameter:
For setting it you should take BuildCollection activity and choose a string type:
Then create a workflow variable of InArgument<Collection<string>> type called Executors
and set it Values as shown:
where values has claims-like view "i:0#.w|DOMAINNAME\\user". Then set a CompositeTask AssignedTo field to Executors.
Similar to that, you can grow a Dictionary by BuildDictionary activity.
For the <Dictionary<string, string> case note that keys set as text, but values are need to be directly set as C# strings: