Check with the Ui_control if the user switch to the dashboard tab with the media video. (in this example MyTab)
Then send the play command to the media player to start playing.
Something like this, not tested.
[{"id":"f541cbd5620db72e","type":"ui_media","z":"92b7e6e4944123df","group":"77e5165.15420e8","name":"","width":0,"height":0,"order":9,"category":"","file":"","layout":"adjust","showcontrols":true,"loop":true,"onstart":true,"scope":"local","tooltip":"","x":810,"y":280,"wires":[[]]},{"id":"d06752b974f2c0f3","type":"ui_ui_control","z":"92b7e6e4944123df","name":"","events":"all","x":260,"y":280,"wires":[["798570dc1d6fc934","14d529575f6744c9"]]},{"id":"302f14e41d5399ac","type":"debug","z":"92b7e6e4944123df","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"true","targetType":"full","statusVal":"","statusType":"auto","x":830,"y":200,"wires":[]},{"id":"798570dc1d6fc934","type":"switch","z":"92b7e6e4944123df","name":"","property":"name","propertyType":"msg","rules":[{"t":"eq","v":"MyTab","vt":"str"},{"t":"else"}],"checkall":"true","repair":false,"outputs":2,"x":430,"y":280,"wires":[["ee08782890b07a9e"],[]]},{"id":"ee08782890b07a9e","type":"change","z":"92b7e6e4944123df","name":"","rules":[{"t":"set","p":"play","pt":"msg","to":"true","tot":"bool"}],"action":"","property":"","from":"","to":"","reg":false,"x":610,"y":280,"wires":[["302f14e41d5399ac","f541cbd5620db72e"]]},{"id":"14d529575f6744c9","type":"debug","z":"92b7e6e4944123df","name":"","active":false,"tosidebar":true,"console":false,"tostatus":false,"complete":"true","targetType":"full","statusVal":"","statusType":"auto","x":430,"y":200,"wires":[]},{"id":"77e5165.15420e8","type":"ui_group","name":"Default","tab":"492f408.019aa4","order":1,"disp":true,"width":"14","collapse":false},{"id":"492f408.019aa4","type":"ui_tab","name":"Home","icon":"dashboard","disabled":false,"hidden":false}]
