How to create function in runtime in Vb6.0

Hello sir,
Have a good day. i m britto working as software programmer in Vb6.0
In VB6.0, i have some procedure like
sub a_on
msgbox"A"
end sub

Sub b_on
msgbox"B"
End sub

dim str as string
'str may be a_on or b_on

str="a_on"
i m planning to call procedure a_on or b_on based on the variable str
how to call the procedure a_on or b_on depending on variable