changing the reference variable value in class statement

I am trying to run an analysis with the variable "histgr" (for histology group) and the values for histgr are 1, 2, 3, 4 and 5. How can I change the reference variable value from 5 (the last value and default reference) to 3 (my desired reference variable value)?

Here is the code I have tried:

proc logistic data=s1rt;
class histgr (ref="3");
model radiation (EVENT=LAST) = histgr;