Display an embedded google map iframe with a marker on a certain latitude and longitude

No need for JavaScript code nor Google API key:

<iframe src="https://maps.google.com/maps?q=35.856737, 10.606619&z=15&output=embed" width="360" height="270" frameborder="0" style="border:0"></iframe>

Just replace the q variable values with your coordinates.

Note: this doesn't allow more than one point.

If you use the "place mode" of the Embedded API you can use coordinates to place a marker:

<!-- New York, NY, USA (40.7127837, -74.00594130000002) -->
<iframe width="100%" height="450" frameborder="0" style="border:0" src="https://www.google.com/maps/embed/v1/place?q=40.7127837,-74.0059413&amp;key=YOUR_API_KEY"></iframe>

working example