Hi @Maria_DK,
Thanks for sharing this with the community.
In case you weren’t aware, Klaviyo has a direct integration with Facebook (including logging lead ad information and syncing custom audiences). That will make the integration process much more seamless. I recommend checking out how to set that up here.
If you wish to pursue the Zapier solution, I recommend reaching out to Zapier to better understand how to select specific custom properties on their end.
I hope that’s helpful and thanks for being a member of our community.