Use the below snippets accordingly to get the 10 digit primary key, and assign it to your primary key field.
db.SaveChanges()
db.Id = tenDigitSubStringFunction(txtID.Text or your identity number)
Or else add your code here so that i can help you in fixing that.
Using JavaScript :
var str = "1234567890";
var res = str.substring(0, 9)
Using C# code
string name = "qwertyuiopasdfghjklzxcvbnmjk";
Console.WriteLine(name.Substring(0, 9));