How to get the UNICODE code of the string entered by the user (character by character), change it and display it?

Question

.NET

The essence consists in "reversible enciphering" of the line entered by the user.
The characters of the encrypted text must be replaced by characters obtained from the original by shifting their ASCII code by a given value. Further, additional characters are placed between the characters of the message, chosen randomly or pseudo-randomly (I don’t understand how to implement this so that it is possible to decrypt the modified set of ASCII codes). All action takes place in the console.
If someone worked with strings, it will be easy. Thanks in advance for your replies.