Hi,
Please navigate to the Table you are using, in this case, it's Email:

Choose the form you need to use:

Please click Switch to classic:

Double-click this area:

You need to configure properties for Rich Text Editor Control:

Please enter the URL of the web resource you created:

This web resource is a Json file that contains multiple properties of your Email Rich Text Editor, you can refer to my Json file:
{
"defaultSupportedProps": {
"font_names":"Brush Script MT/'Brush Script MT', cursive;Calibri/Calibri, Helvetica, sans-serif;Calibri Light/'Calibri Light', 'Helvetica Light', sans-serif;",
"font_defaultLabel":"Brush Script MT",
"fontSize_sizes":"8/8px;12/12px;20/20px;32/32px",
"fontSize_defaultLabel":"20",
"stickyStyle":{"font-size":"20px", "font-family":"'Brush Script MT', cursive"}
}
}

This is my test result:

For more details on how to create a Json file to configure fonts, please refer to this official documentation:
docs.microsoft.com/.../rich-text-editor-control
If this helped you, I'd appreciate it if you'd mark this as a Verified Answer, which may in turn help others as well.
Best Regards,
Frank Gong