Read about it! at it's place on meta.discourse.org
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You need to set the Access-Control-Allow-Origin header for discourse. That's as simple as:
add_header "Access-Control-Allow-Origin" "*";
Read the guide over here for how to intergrate the two together.
/t/<post-slug>/ and is not the last number.
And if you're really looking to build it out in some other kind of framework I'd recommend looking at the original wp-discourse. Or, just load up
/t/<topicID>/wordpress.json?best=number and parse it from there.