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).