Drag a string type onto the canvas and assign a string value to it to use it as a variable. Use the SetValue to change the string further below in your Function.

A string type holds sequences of unsigned 16-bit (2-byte) code points that range in value from 0 through 65535. Each code point, or character code, represents a single Unicode character. A string can contain from 0 to approximately two billion (2 ^ 31) Unicode characters.

The default value of Linx String is not Nothing (a null reference), as it is in .NET, but an empty string (value "") instead.