API Post application/json Line richMenu API

curl

* curl -v -X POST https://api.line.me/v2/bot/richmenu/bulk/link \
-H "Authorization: Bearer {channel access token}" \
-H "Content-Type: application/json" \
-d '{
"richMenuId":"{richMenuId}",
"userIds":["{userId1}","{userId2}"]
}'

apex

JSONGenerator gen = JSON.createGenerator(true);
gen.writeStartObject();
gen.writeStringField('richMenuId', 'richmenu-xxxxxxx');
gen.writeObjectField('userIds', Set<String> strSet);
gen.writeEndObject();
req.setBody(gen.getAsString());