How to implement Server2Server (S2S) postback

[Fwac]

Hi!
We would like to use the Server2Server integration of outbrain (https://www.outbrain.com/help/advertisers/server2server-integrations/), but I am not clear how to implement the call of the postback url at shopify. (Ajax call is blocked by the browser).
Does anyone have experience with this and has a hint for me?
Thank you!