How to Send Email and relate it to Case using REST API SendEmail Quick Action

To use REST API send email quick Action for cases, make sure that you have Email-to-Case enabled in your org.

Salesforce classic: Setup -> Customize -> Cases -> Email-to-Case

REST API URI: /services/data/v53.0/sobjects/Case/quickActions/SendEmail

HTTP Method: POST

Body:

{    

    "contextId": "500B00000082so8",

    "record": {

        "HtmlBody": "<div+style=\"font-family:+Arial;+font-size:+14px;\">Test</div>",

        "Subject": "Sfdcsupport",

        "ToIds": ["003B000000IqL5L"]

    }

}

Sample Request: 

Your output looks like below: Visit you case feed page, you can find the email attached over the case.