saving username password in option.html and loading them automatically

I have created 2 extensions for websites that i do not own and they do not have an api.

when you open the extension, there will be a login screen, but when you close it, you'll be logged out, i want to either save the cookies so when you reopen the extension you'll be logged in or to save the password like lastpass and create an option page, but:

1- I do not want to host the database on my vps, coz i don't want to pay to maintain the extension if i have many users, and i don't want to keep protecting my database from hackers till death :)

2- I do not want to host it on any paid service

unfortunately, lastpass doesn't have an api, so i'm thinking of storing the username and password on the user's pc, and i do not care if he got hacked because it is his responsibility to keep his pc secured and not mine :) any idea or tutorial on how to do it?

and by the way the websites are created using .net framework

You can store information between browser restarts in localStorage.