How can I set a default option for a radio filter?
There isn’t a dedicated default selected option control for the Filter - Radio element right now.
The current supported way is to load the page with that filter already active through the filter URL parameter, for example ?brx_FILTERID=option-value or, if you set a custom URL parameter on the filter, ?category=shirts. Use the actual filter element ID or URL parameter and the exact option value/slug.
See the Query Filters URL parameters section: Query Sort, Filter & Live Search | Bricks Academy
However, there’s already an internal task for this request. We’ll let you know once it’s implemented.