HMR to enclose = in % %

I have a HMR below that copies the UUI and expires from the route-to header.  I need to enclose the = in the expires in % %, as it is not a supported character according to the rfc. 

 

sip-manipulation
name ModRefer
description
split-headers
join-headers
header-rule
name ModReferTo
header-name Refer-To
action manipulate
comparison-type pattern-rule
msg-type request
methods REFER
match-value (<sip:.*user=phone)\?(User-to-User.*>)
new-value $1+";"+$2

 

So i somehow I need to see :

To: <sip:7206783859@10.161.200.120;user=phone;User-to-User=00C80A35313837373436343936%3Bencoding%3Dhex&Expires%=%24>