iRule Syntax

I have an iRule where I am using the set command to create a pool name to a variable. The command is "set hostpool pool_$msrdp". The command works the way it is, but I want to add additional characters to the end of the pool name. For example, the way the command is now, the pool name that is assigned to hostpool is "pool_0A1", where 0A1 is the contents of $msrdp. But I want the pool name to be "pool_0A1_rdp". How would I modify the command to include the "_rdp"?