You will need to install a user script manager extension to install this script.
This userscript uses GM_config to make configuration simple and easy.
After installing the script, go to any Iwara video page. On the left edge of the header, you will now see the Hydrus Network logo. Click this to open the configuration dialogue.
All options other than API key will have default values. If any of these are incorrect for your system, be sure to update them.
If you have created an API key yourself, you can paste it into the labeled box. If not, the script can request one for you.
If everything worked, the API key will appear in the appropriate field automatically. Save the new settings in the script and in your client and you should be ready to go.
If you toggle the "Add categories as unnamespaced tags" option, the page will need to be reloaded in order to update the tags.
Once the script is configured, all you have to do is click the new "Send to Hydrus" button, which appears directly next to the download button. If everything works, the button will update to let you know that the URL was successfully sent to your client.
Note: The script does not work with embedded Youtube videos. Configuration is still available from these pages, but no other functions will work.