There are many free software for making API requests like Postman. I use Insomnia. A matter of preferences and availability depending on your OS, mostly.
Here’s a simple example with curl from command line:
curl -X POST "https://your-discourse.com/posts.json" -H "Content-Type: application/json" -H "Api-Key: YOUR_API_KEY" -H "Api-Username: YOUR_USERNAME" -d "{\"title\": \"Test topic creation with the API\", \"raw\": \"And here's the topic's content\", \"category\": CATEGORY_ID }"