In order to connect NZBHydra to Sonarr/Radarr, you need to have at least 1 indexer set up in Hydra (Sonarr/Radarr will error when trying to connect if none are set)

Open NZBHydra and set up a Usenet and/or Torrent indexes

and also take a copy of your Hydra API key

If you plan to use Hydra for both Usenet AND torrents in Sonarr/Radarr, you need to add it twice, once as Newznab and once as Torznab

On your Sonarr/Radarr use these settings 

Usenet:

Host  http://127.0.0.1:5076/nzbhydra

Input your API key

if you get an error about Category your need to set them up on the NZBget app via your dash 

save and done

Torrents:

To use for torrents you need to set up Jackett and then add the jackett torzab indexers in Hydra

Then use this config 

Host  http://127.0.0.1:5076/nzbhydra/torznab

Input your API key

save and done