I would show you how I would do it in UD and not PU and @adam has kindly documented the steps here:-
https://docs.ironmansoftware.com/dashboard/interaction#setting-component-state
and another example here:-
https://docs.ironmansoftware.com/dashboard/components/element#setting-element-properties-dynamically
I hope this does solve the issue for you
P.S Sorry I re-read the problem…Please have a look at New-UDSelect DefaultValue doesn't work from a Variable - #2 by psDevUK for getting the default value to work.