I've created a Message Custom Field (for edit) with the following default value:
#if ($authcontext.user.inGroup('Clients'))
<div class="infoBox">
Please make sure to select <b>Client Request</b> as <i>Issue Type</i>. You will not be allowed to submit a Work Ticket.
</div>
#end
For some reason the Velocity code is not processed but displayed as a text.
The field is made 'Required', as JIRA prescribes.