location=("http://www.myTestPage.com/") starts the navigation to the new page. Where do you intent for showForm() to be called from? If it's the current page, I don't get why you want to do that?
This will following though I doubt you want to open a new window, yea?
window.open("http://www.myTestPage.com/");
showForm();
document.getElementById("txtEmail").value="[email protected]";
submit();
To Add:
I think you wanted to submit the form to for server-side process and also navigate to the new location at the same time. Few ways to do it:
Submit the form, and let the response redirect to the desired location
Submit the form asyncronously, after that navigate to new page