bigImageUrl from Datapoint (IOBroker)

**Have you read the documentation?**
- [ X] Yes, but it does not include related information regarding my question.
- [X ] Yes, but the steps described in the documentation do not work on my machine.
- [X ] Yes, but I am having difficulty understanding it and want clarification.

**You are setting up gotify in**
- [ ] Docker
- [ X] Linux native platform
- [ ] Windows native platform

**Describe your problem**
Hello,
I would like to display images (bigImageURL) and that also works, but the URL always changes. I therefore always have to get the current URL from the data point. Is that possible or does it always have to be a fixed URL?

My script runs via Java and looks like this:
```

const axios = require("axios");

const url = "https://192.168.178.14/message?token=APE0xxOUPMIExl2";
const bodyFormData = {
title: "Kamera Garage",
message: ")", <<<< IN THIS DATAPOINT IS MY URL
priority: 8,
extras: {"client::display": {"contentType": "text/markdown"}}
};

axios({
method: "post",
headers: {
"Content-Type": "application/json",
},
url: url,
data: bodyFormData,
})
.then((response) => console.log(response.data))
.catch((err) => console.log(err.response ? error.response.data : err));
```