If you want to see your text field multi-line customfields in JIRA like the system field “Description“, it’s very easy. Just create the field (text field multi-line), and in the Field Configuration Scheme of the project change the field Render to “Wiki Style“. This is all!
(Click here to see how to construct your Description Template in Wiki Render format)
If you need to set a Default template inside the field you can take a look to this other post: How to set a default description template in your Jira tickets.
By MrAddon