This script will help you to open redirection URL in new tab after Landing page submission. This will work in embedded iframe as well.
Navigate to Marketing -> Landing pages -> Create Landing Page/ Edit Landing Page -> Step 4 : Page Settings, in Embed script End of Body section place this below code.
Sent Data
<script>
$('#form-submit-button').click(function(e) {
e.preventDefault(); //prevents the default submit action
$(this).closest('form').attr('target', '_blank').submit();
$(this).closest('form').find("input[type=text], textarea").val("");
});
</script>