Open the browser network tab and look at user suggestions when you're adding a mention comment on a Bitbucket PR.

Posting a mention comment

Request - https://bitbucket.org/gateway/api/v1/recommendations

{
    "recommendedUsers": [
        {
            "entityType": "USER",
            "id": "xxxxxxxxxxxxxx",
            "name": "John Doe",
            ......
        }
    ],
}