The <f:enum>  tag from Jenkins Jelly form library has an attribue default that should set the field value for newly created forms.  It does not work because of the bug in the core/src/main/resources/lib/form/enum.jelly source file. As a result the HTML <select> will have no <option> selected which will make the first enum value be used when submitting a form instead of the wanted value.

  <f:entry field="ballColor">
    <f:enum default="YELLOW"/>
  </f:entry>