No, there's no function that could create a user from a form.
The closest you could get is a listener or post-function that could take a healthy guess at a user account to set up, bypass the "needs to be admin" security and adds one based on the issue being submitted, but that would require code that you can't add to Cloud.
A listener listens for events, like "transition" or "create" and can then do things based on the information it gets from that event. A post function runs when an issue goes through a workflow transition, and post functions are mainly for making automatic changes to data. So, yes, you'd edit a workflow to use them. However, I can't tell you which post-function to choose. It won't work. You need to find or write code for a "create user" post function, as JIRA does not have one. But, you cannot install that on Cloud. So you can't do it.
Is there a way to create users from the creation of the issue. Indeed, I added a user picker custom field in the issue creation form. I would like to know if it is possible to create a user from this form (via a link for example) if I realize the user I'm looking for does not exist yet. The goal is to avoid to cancel the issue being created. Thanks for your answer
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