The Karabiner-Elements settings for the key binding on MicroSoftTeams.

teams.json

{
  "title": "browser",
  "rules": [
    {
      "description": "MicroSoftTeams: send by Cmd+Enter",
      "manipulators": [
        {
          "type": "basic",
          "conditions": [
            {
              "type": "frontmost_application_if",
              "bundle_identifiers": [
                "^com\\.microsoft\\.teams"
              ]
            }
          ],
          "from": {
            "key_code": "return_or_enter"
          },
          "to": [
            {
              "key_code": "return_or_enter",
              "modifiers": ["left_shift"]
            }
          ]
        },
        {
          "type": "basic",
          "conditions": [
            {
              "type": "frontmost_application_if",
              "bundle_identifiers": [
                "^com\\.microsoft\\.teams"
              ]
            }
          ],
          "from": {
            "key_code": "return_or_enter",
            "modifiers":  {"mandatory": ["left_command"]}
          },
          "to": [
            {
              "key_code": "return_or_enter"
            }
          ]
        },
        {
          "type": "basic",
          "conditions": [
            {
              "type": "frontmost_application_if",
              "bundle_identifiers": [
                "^com\\.microsoft\\.teams"
              ]
            }
          ],
          "from": {
            "key_code": "return_or_enter",
            "modifiers": {"mandatory": ["right_command"]}
          },
          "to": [
            {
              "key_code": "return_or_enter"
            }
          ]
        }
      ]
    }
  ]
}