So your Employees have some sort of identifier, say EmployeeId (Autonumber) and you want to pass it to the REST service?
You can add a query parameter of type string.
Create a JSON snippet
Create an import mapping based on that snippet.
In the microflow, you use an import mapping to turn the string into a list of objects.
In the list that you want to return, you can filter out the employees with those EmployeeId's
To create JSON from a list of objects:
1. Create a JSON structure that has a sample of the what you would like the JSON to look like. See https://docs.mendix.com/refguide/json-structures
2. Create an import mapping based on that JSON snippet. See https://docs.mendix.com/refguide/import-mappings
3. In the microflow, use an Import Mapping Action. See https://docs.mendix.com/refguide/import-mapping-action