Creating a user registration form
If you are going to distribute this assembly, you are effectively distributing that password.  Tools such as ILSpy can decompile the assembly and the database will be compromised.

The Windows Forms application shouldn't directly connect to a remote SQL server, there should be a service or API layer on the web sever to handle that.

If you need to authenticate / authorize a user, they will need to each have their own logins.

WinForms should act like a thin client, just sending requests to the server and receiving responses.

The web server should receive, authenticate, authorize and execute those requests on the database, and then send a response.