The Streaming APIs do not support SRT and there is no current plan to do so as it's not a Web Internet standard and is unlikely to ever be.
The Streaming APIs support RTMP/RTMPS and WebRTC today, with plans to support RTSP soon.
Some of our customers are using a Nimble server or Vmix to transcode SRT to RTMP and ingest this into the Streaming APIs (Millicast) (which transcodes RTMP to WebRTC). However we do not have experience with these tools ourselves. This is also not our preferred method as any transcoding steps add on latency.