1. There is no ByRef in Basic4android.
2. Strings are immutable objects. You cannot modify an existing string object. What you are doing is you are assigning a new object to the variable. It will not have any effect on any other variables (assuming that it is a local variable).
You can pass a custom type that holds a string:
Type StrRef (s As String)
This will allow you to pass the StrRef object and change its value.
You can also pass an array with a single string.
Last option is to pass a StringBuilder instead of String. Then you will be able to remove its current content and append the new content.