Hello Gaurav,
Defaulting a value means you should be able to see the value before you Save. You said use the same Exit. Means IWO10009. This performs at the SAVE event of the Order. Means you'll not be able to view and edit the value here before you Save. Then how we will be able to achieve the defaulting values? This is not possible with this exit and as I said I'm not aware of any BAdi can perform this. Please tell us if you have some worked-out method.