I have found the following steps to be the fastest way to default text to a predefined field. This has mainly been used with Bugs but I am sure it is extendible to other issue types. The functions referenced are from the Jira Settings - Issues page:
- Create a custom field and default the text
- Create Custom Field
- Select a field type - Paragraph
- Name and Description
- Use the newly created fields "Contexts and default value" to add the default text you want to appear in the predefined field
- You do not need to associate the field with a context - use the default or a screen

- Identify the "Software Simplified Workflow" that is used by the Jira project then copy and modify it
- Workflows
- List all the workflows used by Jira projects and identify the one that is used for your project
- Click Copy
- Name the new workflow and use the diagram editor to view the workflow
- Click on the "Create" transition, click the post function link, and add a post function
- Use the "Copy Value From Other Field" function
- Source - new custom field
- It might not appear in the list so start typing it and it should show up as a selectable field
- Destination - predefined Jira field
- Add
- Move up the post function until it is the first in the list of actions
- Publish the workflow if necessary




- Add the newly created workflow to the Workflow Scheme for the project and associate it with the Bug Issue type
- Workflow Schemes
- List all workflow schemes and identify the one that is used for your project
- Click Edit
- Click Add Workflow and select the newly created workflow from the list of workflows
- Click Next
- Associate to Issue Type "Bug" and save
- Publish the changes to activate the new workflow for the Bug Issue type on your project


I hope this helps.