String Concatenation Within Deployment Process


I have a question regarding to string concatenation during deployment process.
When I deploy a package to a custom directory, i can assign a variable on the field, such as #{install_dir}
What if I want to concatenating the string variables, such as #{install_dir}/#{site_url}? (please see attached screenshot)

The string concatenation is giving me an error which does not allow me to do it this way.

Is there an alternative way to accomplish it? I do not want to hard code the whole string, or hard code the concatenated string in the variable set.

Thanks in advance