How to return user already exist on angular 7 ?

Question

problem

How to return user already exist on angular 7 ?

I work on web api working on asp.net core 2

I check users exist on database 

if user i create exist on database then message will show to me

user already exist

public static bool CheckUserName(string username)
       {
           bool status = false;
          // string constr = ConfigurationManager.ConnectionStrings["conString"].ConnectionString;
           using (SqlConnection conn = new SqlConnection(SQLDAL.ConnectionString))
           {
               using (SqlCommand cmd = new SqlCommand("CheckUserAvailability", conn))
               {
                   cmd.CommandType = CommandType.StoredProcedure;
                   cmd.Parameters.AddWithValue("@UserName", username.Trim());
                   conn.Open();
                   status = Convert.ToBoolean(cmd.ExecuteScalar());
                   conn.Close();
               }
           }
           return status;
       }

so How to create service on angular 7

and how to call it on Onsubmit event