default thread prefix

[1master1]

go to acp->configuration->thread prefixes and add some prefixes,
example
1
2
3
4

and allow them to be used in a forum. try creating a thread in that forum. you will see no prefix by default and you must select a prefix from those 1,2,3,4.
now i want a particular prefix to be set as default instead of "no prefix" example: 3

how can we make it?