Customization of DWA Send action

Can anyone tell me where to add a Javascript confirm when DWA users press the Send action?

I plan to use the CpY() function to warn people when total attachment size is too big.

The DWA 7 Customization Red Paper is excellent (http://www.redbooks.ibm.com/abstracts/redp4188.html?Open) but I need more granular help in how to modify the code behind a specific action - in this case the memo ‘Send’.

Thanks,

Peter.