I'm afraid that you'll need to add that button.
The only way that you can trip a script trigger in IWP is with another script. user interaction with their web browser won't trip a trigger.
This is one of the issues corrected with the release of WebDirect with FileMaker Server 13.
I had an idea, maybe. The way it works now, is my customer selects their customers name, the commit populates the address.
I can set up related records and when my customer selects their customers name...(from a value list) can/will/does that trigger it to populate the address in IWP?
Only once the data is committed back to the server.