You need to look for the following in commands.cfg:
Code:
[Cmd561]
Name=Toggle_Activate_UserRadio
Contexts=1
Then add a line just below, defining the key combination you want to use.
In the following format:
Key0=0x52,c,"CTRL+R"
ASCII Code
s or c for SHIFT or CTRL
The part in quotation marks is just a description to make the file more human readable.
Here's an ASCII table:
http://www.asciitable.com/
Use the values listed under the heading 'Hx', and prefix them with 0x
Certain keys and combinations just won't work, so you may have to experiment. When it works, you'll know it

If you use the example above, CTRL+R will bring up the radio.