You can't do that from within ASPX. Targeting frames is only possible on
client side.
Either, use standard Anchors (<a target=''...>) or add a client-side
JavaScript portion to your original page which is only executed on PostBack
if you want to update other frames after some form submission:
1. Use frame index. This will open a page in the second frame.
window.top.frames[1].location = url;
2. Use frame id. This will open a page in the frame that has id="mySecondFrame".
I personally prefer this approach because you won't have to modify your code
if you reorganize the layout of your frames in the future.