@ben - Maybe you've already answered this question, but here we go...
I don't think JSM can do exactly what you have described.
Can you explain why you want pre-defined text in your description field? Do you want to prompt portal users to answer specific questions or provide specific information?
If you want to prompt the portal user to answer questions you should check out JSM Forms which are designed to do this. This also allows you to make fields conditional and interconnected.
If you don't want the portal user to see the pre-defined text you could add text to the description after the request is created using Jira Automation. We have used Automation in JSW and JSM to populate or set fields after issue creation.
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