How To Configure PfSense IPSec but PfSense is not as default gateway

[The Predator]

Hi everyone,

I have 2 sites need to be connect with IPSec, Site A using PfSense as a default Gateway. Site B using ISP modem router as a default gateway, PfSense is located after ISP router. I already configured OpenVPN and it's working at Site B. I can access it from outside.

My problem is I dont know how to setup IPSec tunnel from Site B to Site A. Site A already have IPSec tunnel connect to other sites.

Do I need to configure something on ISP router before configure IPSec at PfSense at Site B?

Please help :(

Thanks!