This is not clear. Please consider adding some detail!
You can use Behaviours for this:
https://scriptrunner.adaptavist.com/latest/jira/recipes/behaviours/setting-default-fields.html
You can also add a post-function on the create transition that sets the priority, but this means you can't edit the priority when creating the issue:
https://confluence.atlassian.com/jira064/how-to-update-custom-field-values-during-workflow-transition-720415925.html
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