The OnObjectModify script trigger fires when the object is active, has focus and then the field is modifed. Do you are a field that the url is keyed into? I would suggest using the script trigger on this url field. The webview can not have focus if a url is being enter in another field.
Except for the initial page, I don't set it programmatically. The initial page is loaded and the user navigates pages within the viewer. I was hoping when the url changed by clicking a link within the viewer it would trigger a modify event. It seems to ignore all events save for entering and exiting the web viewer.
Found a workaround though since it looks like the web viewer runs in a separate process from FMP. On enter, I install an ontimer script that runs every half second and gets the current url, along with subscripts to perform actions based on that url. on exit, I turn off the timer. At the end of the script I save the url in a global variable so when it next runs it can compare to the current url and exit immediately if it's the same.