Not understanding D-ID and Elevenlabs custom API

Hi there -

I'm in a trial or free tier of D-iD while I learn if all will be what I need. So, not sure if that's my issue or what.

I'm following this page: https://docs.d-id.com/reference/tts-elevenlabs

I have my own custom voice on Elevenlabs. And I'm new to JSON.

Partly, I don't know what url to use for Post. My understanding from the doc I point to above is it should be:
https://api.d-id.com/talks?x-api-key-external="{"elevenlabs": "MY11LABSAPIKEY"}" but obviously I can't use curly brackets in a url string. So it's a super dumb question but what should the post url be?

If I remove those parameters and just use /talks, and use the x-api-key-external etc. as the key and value in my header, I get {
"kind": "UnknownError",
"description": "Internal Server Error"
}

This is what my JSON body looks like:

{
"script": {
"type": "text",
"subtitles": "false",
"provider": {
"type": "elevenlabs",
"voice_id": "sJxcrPKArpLZzS8qpgxh",
"voice_config": {
"stability": 0.3,
"similarity_boost": 0.75
},
"model_id": "eleven_multilingual_v2"
},
"ssml": "false"
},
"config": {
"stitch": "true"
}
"source_url": "MYDOMAIN/SOMETHING.jpg",
}

Any coaching on just how to post to create a video with my custom Elevenlabs voice is appreciated. Even if I need a paid D-iD account, no problem, but I still don't know what the post url/parameters should be. Thanks.