When you say template, do you mean a custom project template of the configuration (eg. workflows, fields, etc) to use during project creation?
If yes, it's not possible to add a new template into the project creation screen - but you can choose to share configuration with a pre-existing project.
As a Jira Admin:
^ This way, you can reuse the same workflows, screens, etc from one project to another. Your first project could be the "template".
Note: This is not available with the Free version of Jira.
Let us know if this isn't what you meant so we can assist further :)
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