Example:
curl -H "Content-Type: application/x-isdoc" -k -u xxx:xxx -X PUT https://localhost:5434/c/firma/faktura-prijata?typDokl=code:ZBOZI-FAKTURA" -T "/cesta/soubor.isdoc"
-u specifies the authorization credentials for Flexi.
-H header specifying the file type
-k if you are using a custom installation with an automatically generated certificate, you must ignore the untrusted certificate authority.
-T the file being sent (request body)
cURL has its own documentation.