Your requirement is not so clear to me. I guess you want to utilize the backup ISP link for incoming traffic to your client location.
All inbound traffic depends upon your BGP network advertisements.
So, you need to have a dedicated Public registered network (/24 at minimum) assigned to your remote location so that you can advertise this network through backup AS with higher preference (like no AS-path prepend through backup ISP and AS path prepend for the same network through primary ISP). Please note that advertising /24 network will not work if any upperstream ISPs are aggregating the customer routes to reduce the entries in BGP database but I have seen /24 subnets as well in BGP routes from several providers.
With best regards...