In WebAPI POST method how to pass Id which is set to Uniqueidentifier in database ASP.NET?

Question

I have a Users table in database, its Id PK is set to Uniqueidentifier. Now using the Web API I am adding a new user through postman into database. when I add the first record it adds the user into database but for adding further records its not allowing me to add because a unique id is not getting generated. On internet I have found that I can use GUID for that purpose but unable to find that how I can pass it in my controller using Linq. here is my Post method for adding new user that I am using:

[HttpPost]
public async Task<IHttpActionResult> AddNewUser([FromBody]  User user)
{              
    try
    {
        using (var newuser = new hospiceEntities())
        {
            newuser.Users.Add(new User()
            {
                id = user.Id,
                UserName = user.UserName,
                PasswordHash = user.PasswordHash,
                Discriminator = user.Discriminator,
                ApplicationId = user.ApplicationId,
                LoweredUserName = user.LoweredUserName,
                IsAnonymous = user.IsAnonymous,
                Email = user.Email,
                IsApproved = user.IsApproved,
                IsLockedOut = user.IsLockedOut,
                CreateDate = user.CreateDate,
                FailedPasswordAnswerAttemptCount = user.FailedPasswordAnswerAttemptCount,
                IsTempPassword = user.IsTempPassword,
                LockoutCount = user.LockoutCount,
                AccessFailedCount = user.AccessFailedCount,
                LockoutEnabled = user.LockoutEnabled
            }); ;
            newuser.SaveChanges();
            return Ok(Helper.SuccessResponse());
            // await newuser.SaveChangesAsync();
        }
    }
    catch (Exception ex) {
        return BadRequest(Helper.ErrorResponse(ex.Message));
    }    
}

The first record being entered into database is as below:

Answer 1

try this

try
    {
        user.Id=Guid.NewGuid();

        using (var newuser = new hospiceEntities())
        {
            newuser.Users.Add(user);
            
           await newuser.SaveChangesAsync();

            return Ok(Helper.SuccessResponse());
           
        }
    }
    catch (Exception ex) {
        return BadRequest(Helper.ErrorResponse(ex.Message));
    }    
}