Samples/hello-world/outlook-hello-world/taskpane.html

<!-- Copyright (c) Microsoft Corporation. All rights reserved. Licensed under the MIT License. -->
<!DOCTYPE html>
<html>

<head>
    <!-- Office JavaScript API -->
    <script type="text/javascript" src="https://appsforoffice.microsoft.com/lib/1/hosted/office.js"></script>
</head>

<body>
    <p>This add-in will insert the text 'Hello world!' in a new message.</p>
    <button id="helloButton">Say hello</button>

    <!-- The following image URL tracks diagnostic data for this sample add-in. Please remove the image tag if you reuse this sample in your own code project. -->
    <img src="https://pnptelemetry.azurewebsites.net/pnp-officeaddins/samples/outlook-add-in-hello-world-run" />
</body>

<script>

    Office.onReady((info) => {
        if (info.host === Office.HostType.Outlook) {
            document.getElementById("helloButton").onclick = sayHello;
        }
    });

    /**
     * Writes 'Hello world!' to a new message body.
     */
    function sayHello() {
        Office.context.mailbox.item.body.setAsync(
            "Hello world!",
            {
                coercionType: "html", // Write text as HTML
            },

            // Callback method to check that setAsync succeeded
            function (asyncResult) {
                if (asyncResult.status ==
                    Office.AsyncResultStatus.Failed) {
                    write(asyncResult.error.message);
                }
            }
        );
    }

</script>

</html>