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This will depend whether you want to use Orchestrator and maybe also whether you are willing to wait for the new functionality to be fully implemented.
For user input, there are several options. I would suggest playing around with UiPath.Form.Activities package which should soon reach Stable release (currently in beta, see here).
For starting your process, in general, your options are:
- Use Orchestrator and Orchestrator API to start your process. You will want to send the start job request and Orchestrator should launch the process as soon as your current one finishes.
- Wait for the Long Running Workflows functionality to be fully implemented (the template for it is already available in Studio 2019.9.0-beta).
- The easiest, mentioned by @Palaniyappan. I forgot the most obvious activity called Start Job