You referring to the yellow bracket or the spigot spike? These can be attached to the seat base just like the stock seat and is how Corbin themselves attach the R3 Solo Classic, Fast Gun and Young Gun versions.
If you mean the spigot spike, some have re-used the one out of their stock seat. The Corbin ones have a thread and screw in/out. There is a squared base for a thin 'C" spanner but a shifter will probably do the job too.
I think the stock ones are riveted on and could be riveted or screwed on with a self tapping thread into the hard molded plastic or resin Corbin seat base. Many people use the Triumph Dual Classic seat with only the front spigot engaged and do not even bother to mount the rear frame bracket and spigot which replaces the stock pillion seat frame bracket..