What are the available preset_name values for subtitles object?

I'm using /v2/video/generate endpoint and trying to customize subtitles. When I send:

{
"caption": true,
"subtitles": {
"font": "Bebas Neue",
"font_size": 96
}
}
I get error: "subtitles.preset_name is invalid: Field required"

Questions:

What are the valid preset_name enum values?
What is the complete JSON schema for the subtitles object?
Can I customize font/size/color/position, or only use presets?
The documentation at /reference/create-an-avatar-video-v2#subtitles-object doesn't show the subtitles object parameters.

Thank you!