Hi Niloofar:
LabKey assay designs are a combination of several domains, and there isn't an API for creating that assay design itself. You can create the design using the UI (simplest, and covers a great many use cases) and then add runs/batches using the experiment.saveBatch API, though that also isn't as straightforward as using the UI or simply file import automation to add assay runs and data. Learn more here:
Another option if needed is to add a file-based assay design in a module, though that would likely involve more support than I can provide here. You can learn more about what's involved in an assay design in the documentation here:
I know my colleague Jason also reached out to you by email, so I'd be happy to follow up there as needed.
Thanks,
--Molly