Can we have a specific facet by default selected for the first time when the rec

You may use default filters. But that will remain applied all the time.

No, i dont want that filter applied all the time. For the first time one of the User Filters should have a default value so later they can click on the cross mark on the filter option to look for other filter options.

A service-backed record might allow you do do this by pre-populating the rsp!query object in some way, but it's really quite complex. More information here: forum.appian.com/.../Service-Backed_Record_Tutorial.html

I tried that, but any updates in rsp query changes the object type to text. Moreover I cannot find a logic to undo that pre-population of field.

I was more thinking of returning a different rsp!query object if the "real" rsp!query object filters are unpopulated, that way you'd effectively be faking a user clicking one of the filters. It might, however, be difficult to handle the user removing that filter.