I found a working solution for those using Castr.io to distribute SRT stream.
Duplicate the existing YouTube SRT (Beta) service and only change the items show below:
<LiveStream serviceName="YouTube SRT (Beta)" url="
srt://southafrica.castr.io:9998" key="r" streamIdExtensions="ff6f82408c7d22eca72c791fd332f8199" lowLatency="False" streamDownConvertMode="None">
<bitrate low="6000000" high="9000000"/>
<audioBitrate low="128000" high="128000"/>
</LiveStream>
For the streamId you only copy anything after 'r=' and before the comma.
#!::r=ff6f82408c7d22eca72c791fd332f8199,m=publish