As @outrun said you can use the Alexa HomeSkills API to trigger intents without an invocation word by using one of their pre-defined commands.
The problem is unlike a custom intent it does not allow you to provide a URI to call upon invocation. Instead you have to use an AWS Lambda. This means you can’t just forward the intent to Home Assistant like you can with a Custom Skill.
It’s probably possible to create a Lambda that proxies the call to the HomeAssistant API, similar to what is described here, AWS Lambda Proxy Custom Alexa Skill when you don't have https
Has anyone tried this or got a working example of proxying the call from a Home Skill via Lambda to a Home Assistant HTTPS endpoint?