I only want to offset bet after my opening bet is filled (matched)

There are 3 options available when offsetting any bet.

Offset placed at same time as opening bet
Offset placed after opening bet is filled
Offset placed as opening bet starts to get filled

For the 2nd and 3rd options above and to have the offset placed once the opening bet is filled/starts to be filled the 'Fill or Kill' option must be enabled in your Global Settings. 

How much of the bet you want to have filled before starting the offset process is controlled by the number of Batches used. Foe more information on Batches please see the following page of our user guide.