C# Concatenate strings or array of chars

I'm facing a problem while developing an application. Basically, I have a fixed string, let's say "IHaveADream"

I now want to user to insert another string, for my purpose of a fixed length, and then concatenate every character of the fixed string with every character of the string inserted by the user. e.g. The user inserts "ByeBye" then the output would be: "IBHyaevBeyAeDream".

How to accomplish this?

I have tried with String.Concat and String.Join, inside a for statement, with no luck.