You’ll have to perform two steps: add a Jira API token and a custom field value to the template using this instruction: How to add a custom field value to the template?.
How to create a Jira API Token?
Go to Account settings.
2. Click a “Create and manage API tokens” link in the Security tab
3. Click Create API token button
4. Enter an API token name and click Create
5. Make sure you copy your new API token. You won’t be able to see the token again.
6. Enter the token into Mail Me > Tokens > Jira API token, and click Validate & Save.
How to add Insight custom field values?
This topic is fully covered in a How to add a custom field value to the template? page.