To achieve that, you'll need to use VL and script :
1) Create an empty group and 2 text object (1 onair, the second not visible)
2) Create a VL to link PosY of this group to Text of on air text value (a format block, or trunc block will be necessary to get an integer).
3) Put a key at frame 0 with PosY of your empty group at 0 in an animcontroller of your main SD).
4) Create a script into OnOnline that will set a keyframe on your animcontroller at the end of your animation (Set PosY of empty group to the value of the second text object).
Value of your on air textobject will increase with your animcontroller...
I hope you'll understand me...
#XPression