You can set a description by using the default assignee field by creating a new field configuration scheme and field configuration then set the field configuration to the applicable project.
Not 100% what the background and usage but another option is to create a new user picker custom field if you need the signal for "assignee" to be more clear when creating the issue then create a post-workflow function to copy the user from that custom field to assignee then set the appropriate field visibilities on-screen.
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