Adding brackets for appended CRM email tracking token in the subject line

Hi all, 

Is there a way to add a brackets around the tracking token? Emails that are currently tracked in CRM are appended with CRM:000000 in the subject line. It is sort of confusing to the recipients/target audience. We would like the tracking token to be xxxx [CRM:0000000] 

Is this possible? I was only able to add the open bracket ([) to the Prefix but was not able to add the close bracket (]). 

Any suggestion is appreciated! 

Thank you
 
Regards,
Sue