Create a script file, e.g. /var/nextcloud/.scripts/my-curl-script.sh which contains the desired curl command. As usual use $1, $2, $3, ... to access the command line options which you’ve parsed to the script.
The work flow app use the full path to your script and use, e.g. “%n” to parse the relative nc file path, like e.g.:
/var/nextcloud/.scripts/my-curl-script.sh "%n"
Keep in mind that “%n” only contains the relative file path from the Nextcloud data directory, e.g. “/user/files/YourDirectory/20191111_184052.jpg”.