Create action for baseline installation with Pre and Post actionscript using REST APIs

I want to store some custom information while submitting action. Based on RFE 99300 , IBM has recommended to use MIMEField while creating action and store custom info using MIMEField.

I am creating action using type .

<BES xmlns:xsi="" xsi:noNamespaceSchemaLocation="BES.xsd">
<CustomRelevance><SOME RELEVANCE></CustomRelevance>

I checked in BES schema where MIMEfield element is not associated with SourcedFixletAction.
So I think I can’t store MIMEfield while creating action.

Please correct and guide me to store information while creating any action.