Hi Rafael,
This should do the trick.
My commands where:
tiddlywiki test --init server
tiddlywiki test --listen host=<yourIP>
In a different terminal:
curl -X PUT -i 'http://<yourIP>:8080/recipes/default/tiddlers/New' --data '{
"tags": "FirstTag [[Second Tag]]",
"my-custom-field": "Field value",
"text": "your text comes here!\n\nnext paragraph"
}' -H "X-Requested-With: TiddlyWiki"
The info missing in the link was the X-Requested-With: Tiddlywiki header.
Be aware, that there is no username password if you start TW with the default --listen command.
I needed to use a linux terminal on a windows machine for the curl command. That's why I needed <yourIP>. If you work on a unix system you can use localhost instead.
have fun!
mario