Handling Parameters for a Custom URI Scheme

I am trying to create a custom URI scheme to launch Remote Assistance (msra.exe) the command to launch the applications is

"C:\windows\system32\msra.exe" /offerRA "%1"

In this command the %1 needs to be replaced with the machine name. HTML tag looks like this

<a href="msra:machineName">Remote Assist</a>

When I click the link it does launch msra but it errors out because it can't find the machine. I built a little console app to see what is getting passed into my command as a paramete and %1 = msra:machineName.

How do I construct the link so that I can extract just the machine name and pass that to the command?