can we have save draft button in start form?
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No. Saving a draft means that the rule input of the form are persisted into the AC variables of the user input task in an active process instance. As the start form only initiates the process on submit, there is no place where to store the data.
What is your goal?
My goal is to give save draft option in very first interface of the site
The option is only available on User Input Tasks. There would be nothing to save on a Site interface (unless you're talking about a task within a process, in which case it's easy).
Typically in this case, we have a User Input task inside of the process, which references the same form. A Save Draft button will submit the start form and route to this task, which essentially saves the data in their tasks list, seamlessly to the user. Be sure to apply escalations and other task cleanup activities on this node as it does create a running process instance.