SubString in MVC Core using html and not razor

I am new to mvc core please assist with the following scenario. I have an employee form with Id(Primary Key) column and Identity number column, I am supposed to get the primary key from the first ten digits of the identity number, when my form saves the first ten digits must go to the primary key field. My view is in html and not razor. Please assist me.I am new to mvc core please assist with the following scenario. I have an employee form with Id(Primary Key) column and Identity number column, I am supposed to get the primary key from the first ten digits of the identity number, when my form saves the first ten digits must go to the primary key field. My view is in html and not razor. Please assist me.