Universal Media Player select_source command and Alexa

Hi all.

I have a Universal Media Player working well with Alexa over the Nabu.
Alexa voice commands such as turn on/turn off/set volume/mute all working fine.

Could someone please tell me what Alexa voice command will trigger select_source command of the Universal Media Player?
Thanks.

I have a similar question because I have an mqtt media player device in HA, but cannot find a way to get Alexa to send the select_source command. Even if I create routines in Alexa for each possible source and that have them call an HA script with the source name, but can’t find how to call the script with a parameter.
I have considered creating my own Alexa Custom Skill but would want it to use the HA Cloud send commands to my HA instance and not sure that is possible either.
Any suggestions would be appreciated.