Please see details on using the Telegram API:

Telegram Linking

JSON for a basic post with a link and image to Telegram:

Telegram Post
{
  "post": "The best Telegram message ever #best https://www.telegram.com",
  "mediaUrls": ["https://img.ayrshare.com/012/gb.jpg"],
  "platforms": ["telegram"]
}

Telegram will automatically preview the link in the channel or group unless there is an image or video included. In the above example the image will show. Removing the image will cause the link preview to show.

If your video doesn’t end in a known video extension such as mp4, please use the isVideo parameter. See the /post endpoint for details.

Animated GIFs

Only one URL is allowed with a Telegram animated GIF. If the media URL does not end in “.gif” or “.GIF”, set the isVideo field to true.

Telegram Animated GIF
{
    "randomPost": true,
    "platforms": [
        "telegram"
    ],
    "isVideo": true,    // Set to true if the mediaURL does not end in .gif or .GIF
    "mediaUrls": ["https://img.ayrshare.com/012/cat.gif"]
}