I was able to use message card itself for inserting HTML in the way below, the HTML went as a value against the key "text" and I made it a separate group under sections
{
"@type": "MessageCard",
"@context": "http://schema.org/extensions",
"themeColor": "0076D7",
"summary": "XXXX",
"sections": [
{
"activityTitle": "XXXX",
"activitySubtitle": "",
"activityImage": "",
"facts": [
{
"name": "XX",
"value": "**1**"
},
{
"name": "YY",
"value": "**test**"
}
],
"markdown": true
},
{
"startGroup": true,
"text": "<table bordercolor='black' border= '2'><thead><tr style = 'background-color : Teal; color: White'><th>Task</th><th>Status</th><th>Start Time</th><th>End Time</th></tr></thead></thead><tbody ><tr><td>xxx</td><td>yyy</td><td>14:25</td></tr><tr><td>xxx</td><td>yyy</td><td>15:25</td><td>16:25</td></tr></tbody></table>"
}
]
}