Where can I find System.TeamProjectId for a project in Azure Devops

You can examine the HTML in the UI of the dashboard on your organisation projects page:

You can get the all team projects id with REST Api:

 https://dev.azure.com/{organization}/_apis/projects?api-version=5.0-preview.3

Results:

 {
  "count": 3,
  "value": [
    {
      "id": "eb6e4656-77fc-42a1-9181-4c6d8e9da5d1",
      "name": "Fabrikam-Fiber-TFVC",
      "description": "Team Foundation Version Control projects.",
      "url": "https://dev.azure.com/fabrikam/_apis/projects/eb6e4656-77fc-42a1-9181-4c6d8e9da5d1",
      "state": "wellFormed"
    },
    {
      "id": "6ce954b1-ce1f-45d1-b94d-e6bf2464ba2c",
      "name": "Fabrikam-Fiber-Git",
      "description": "Git projects",
      "url": "https://dev.azure.com/fabrikam/_apis/projects/6ce954b1-ce1f-45d1-b94d-e6bf2464ba2c",
      "state": "wellFormed"
    },
    {
      "id": "281f9a5b-af0d-49b4-a1df-fe6f5e5f84d0",
      "name": "TestGit",
      "url": "https://dev.azure.com/fabrikam/_apis/projects/281f9a5b-af0d-49b4-a1df-fe6f5e5f84d0",
      "state": "wellFormed"
    }
  ]
}

You don't even need use Postman or create Http request, just enter the API url above in the browser.