If you want to redirect the parent page from iFrame, use javascript:
top.location = '<your redirect url>';
If you want to redirect from parent to iFrame, use JavaScript:
document.getElementById("iframeId").src = '<your redirect url>';
If you are looking for something else, please help me understand your question.
you can add iframeId like:
<iframe id="iframeId" src="http://127.0.0.1/SignIn/docs/index.php/formbuilder/frontend/viewform/?form=2" style="border: 0; height: 100%; left 0; position: absolute; top: 0; width: 100%;">
</iframe>