How can I set up WBS numbering as below in microsoft project?

100 One type of test
> 101 test 1
> 102 test 2
> 103 test 3
> 104 test 4
200 Another type of test
> 201 test 5
> 202 test 6
> 203 test 7
> > 203.1 (First bit of test 7)

All I can work out how to do so far is:

100
> 100.1
> 100.2


> 100.3 etc...