Replacing a substring in an xml node with a 'Set Value'

I have a node that looks something like this:

<EmailBody> &lt;p&gt;Ticket {$Tckt_Cd$}&lt;/p&gt </EmailBody>

I want to replace {$Tckt_Cd$} with a variable in the Workflow service. I'm looking for help on this one.

I'm using LC ES 8.2