Hi @fxholl, n8n items are an array of 0 or more JSON objects, and these square brackets are how arrays are represented in JSON notation.
When using the HTTP Request node, n8n will only send a single item in each request, not the whole array. So, there should be no square brackets surrounding your item.
The problem in your case will most likely be that n8n nodes are rather picky as to what data format exactly they accept. For some nodes, a string representation is expected, for other nodes an actual JSON object is required. This is unfortunately inconsistent, so when in doubt you might need to try both (for the HTTP Request node a string is required, but it would still need to represent a valid JSON structure). In addition, line breaks can interrupt expressions.
So, in this specific case you want to use a stringified JSON object and no line breaks. For example like so:
This request would arrive on the receiving server like so:
Hope this helps!