create unique id(custom id ) in sql

Question

I want to automatically generate unique id (custom id)

for example: If a applicant register then custom id created like "FM01" and i am create this in model of Asp.net MVC Core

[key]

public int UserId { get; set; }            

[Required(ErrorMessage ="Please Enter Username..")]  

 [Display(Name = "UserName")]  

public string Username { get; set; }