Current behaviour:
It is not possible to add Wiki markup to the descriptions of the custom field and show them on the portal. It is possible to add wiki markup to the custom field added to the issue view:

Expected behaviour:
It would be great if this feature is also provided for the descriptions of custom fields added to the Request Type.

Use case:
This is very useful in cases when the description needs to be highlighted to the customers.

Workaround:
Use https://jira.atlassian.com/secure/WikiRendererHelpAction.jspa?section=all