Hello Injuda,
Hope you're doing well.
The Radio Group widget has a Variable property where you should specify the variable that holds the value selected by the user:
The Radio Button widget has a Value property where you should specify the value that corresponds to that option:
In order to select a default value, you just need to assign the value of your default option to the variable that is bound to the Radio Group.
You can do that for example in the OnInitialize event of the screen / block.
Hope that this helps you!
Kind regards,
Rui Barradas