Specify a start point in a Page Selector field type

I have a custom "Page Type" with a "Page Selector" field in it. I would like to set a kind of "start point" for the user to select a page. For example: - When the user is editing a page (with that Page Type) and click on "Select" button for that field, a "Select Path" modal window appears with the root page selected (in left panel). I would like to let a specific page set (in the document tree, in left panel) instead of the root document so that the user could select child node of that page I pre-set.

The goal is to facilitate the things to the user because, for that specific field, the user can only select pages under a specific node in the document tree.