Here is a simple example showing how to use an external font in TinyMCE.
Note, to run this example, make sure to add your API key in index.html where you see YOUR_API_KEY_HERE appears in the script call to load TinyMCE.
Please note, that configuring the list of fonts available in the fontselect dropdown is a separate task. For information on that, visit: https://www.tiny.cloud/docs/configure/editor-appearance/#font_formats