Simple API Post to update variable

I want to update a flow context variable through an external API call. For this I used the Http In node configured as [POST] for /myapi/myvar.
Then I launch a curl call like:
curl http://myserver:1880/myapi/myvar -d "myvar=true" (I also tried --data=true)

And somewhat I expected to get "true" in my payload. But instead I get { myvar: "true"}
Is it something with the curl command? Is it possible to directly get the value in payload?

Ok, I found myself. The curl format should be something like:
curl -X POST -d 'true' -H "Content-type: text/plain" http://myserver:1880/myapi/myvar