There is a Rest API could update work item filed.
For example:
PATCH https://servername:8080/tfs/DefaultCollection/_apis/wit/workitems/[testcaseid]?api-version=1.0
Content-Type: application/json-patch+json
Body:
[
{
"op": "add",
"path": "/fields/Microsoft.VSTS.TCM.AutomatedTestName",
"value": "[namespace.classname.methodname (e.g. UnitTestProject1.UnitTest1.TestMethod2)]"
},
{
"op": "add",
"path": "/fields/Microsoft.VSTS.TCM.AutomatedTestStorage",
"value": "[assembly name(e.g. unittestproject1.dll)"
},
{
"op": "add",
"path": "/fields/Microsoft.VSTS.TCM.AutomatedTestId",
"value": "[guid id]"
},
{
"op": "add",
"path": "/fields/Microsoft.VSTS.TCM.AutomatedTestType",
"value": "Unit Test"
},
{
"op": "add",
"path": "/fields/Microsoft.VSTS.TCM.AutomationStatus",
"value": "Automated"
}
]
The AutomatedTestId is a Guid value, so you can generate a new Guid by using this C# code:
Guid g = Guid.NewGuid();
string s = g.ToString();
Take a look at this similar question How do I associate test methods to test cases?