You must change the Body field to full Expression mode:
1- In the HTTP node, in the Body Parameters section, change from “JSON” to “RAW Parameters (JSON)”.
2- In the Body field, click the gear icon and select “Add Expression”.
3- Write the full expression as a valid JavaScript string:
Thanks but I still have an error 400 with your solution. I guest in the content type I’m supposed to write application/json, right ?
Small detail, I’m hosting n8n at home (not sure if it changes something but just because I don’t have the “RAW Parameters (JSON)” option and only RAW…).