How to make a post with a from data of empty json through HTTPie?

Use <<< operator like below.

http POST ooxx.asdf/ Content-Type:application/json <<< '{}'

Verbatim request data can be specified via redirected STDIN:

$ echo '{}' | http httpbin.org/post

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Note that for requests that include a body:

• POST is the default HTTP method
• application/json is the default Content-Type