Okay, if you want a button, add a button. No problem.
But if you want to use the button for something, you need to know what for and design a solution that meets the goal. The usual point of this is to allow users to a change a query that is used to fetch and process some records. For example, RSX wanted something "similar to the one that we have in Purchase order > Purchase > Generate > Purchase inquiry", where it's used to fetch information about orders and generate records in PurchParm* tables.
But I have no idea what you want to use it for. You may, for example, use to modify the query used by the form data source, which is a completely different scenario and it would be implemented in a different way. Knowing the requirements is really crucial because there is no universal implementation for all possible requirements.
In general, the answer to 'How to get Standard "Select" query button in custom form?' is that there is no standard "Select" button. The standard way is using SysOperation (or RunBase) framework. But you surely can implement your own "Select" button doing anything you like. What - that depends on your requirements. If you don't want to use the frameworks and just open query ranges, you need somehow obtain some query, create QueryRun object and call its prompt() method. And do something with the result if the user confirms the dialog.
