Validate model after adding updating model - authentication

Question

I am working on a web api on .net core. I am trying to validate/revalidate a model after adding additional data into the model (namely after adding the currently logged in user's username).
Let's say this is my controller's action
[Authorize]
[HttpPost]
public void Update([FromBody]UpdateUser User)
And here is the code I used for my authentication Scheme:
string token = Context.Request.Query["token"];
if (token == null) return AuthenticateResult.Fail("No JWT token provided");
try
{
var principal = LoginControl.Validate(token);
return AuthenticateResult.Success(new AuthenticationTicket(principal, SchemeName));
}
catch (Exception)
{
return AuthenticateResult.Fail("Failed to validate token");
}
Basically I am trying to achieve this flow:
Post request -> Authorize-> add the user ID into the model -> validate the model.
So after authorized, I need to first add User.UserName = CurrentUserName and only the model needs to be validated, after which I can use the ModelState object on the newly updated model.
As of now I am trying the following:
[HttpPost]
public async Task Update([FromBody]UpdateUser User)
{
User.UserName = "hello";
bool valid = await TryUpdateModelAsync(User);
valid = TryValidateModel(User);
}
Right now at both instances valid is false and the ModelState shows that UserName is required. The only validation I had added in UpdateUser is adding [Required] in the model.

Answer 1

I got it working by clearing the model before calling TryValidateModel.
[HttpPost]
public async Task Update(UpdateUser User)
{
User.UserName = "hello";
ModelState.Clear();
TryValidateModel(User);
// ModelState is now reset
}