Having something like the following would work.
<iframe id = "testframe" onload = populateIframe(this.id);></iframe>
// The following function should be inside a script tag
function populateIframe(id) {
var text = "This is a Test"
var iframe = document.getElementById(id);
var doc;
if(iframe.contentDocument) {
doc = iframe.contentDocument;
} else {
doc = iframe.contentWindow.document;
}
doc.body.innerHTML = text;
}