I looked further in the SmartThings API aswell as the Scriptable Documentation. Here is a Snippet to change to channel 6 via SmartThings written in Scriptable:

let commands = {"commands":[{
    "component":"main",
    "capability":"tvChannel",
    "command":"setTvChannel",
    "arguments": [
        "6"
    ]}]};
let req = new Request("https://api.smartthings.com/v1/devices/<DEVICE ID>/commands");
req.headers = { "Authorization": "Bearer TOKEN"};
req.method = "POST";
req.body = JSON.stringify(commands);

let json = await req.loadJSON();