You can add @mentions within an Adaptive Card body for bots and message extension responses. like below-
If you want to use FactSet please refer the below sample-
https://github.com/OfficeDev/Microsoft-Teams-Samples/blob/391b4de9daf91e6b1befa73214b04ad4736f36a1/samples/bot-sequential-flow-adaptive-cards/csharp/SequentialUserSpecificFlow/Resources/rejectedCard.json
{
"contentType": "application/vnd.microsoft.card.adaptive",
"content": {
"type": "AdaptiveCard",
"body": [
{
"type": "TextBlock",
"text": "Hi <at>John Doe</at>"
}
],
"$schema": "https://adaptivecards.io/schemas/adaptive-card.json",
"version": "1.0",
"msteams": {
"entities": [
{
"type": "mention",
"text": "<at>John Doe</at>",
"mentioned": {
"id": "29:123124124124",
"name": "John Doe"
}
}
]
}
}
}