how to concatenate a variable with a macro?

For example, I have a variable $str1="hi" and a macro ${str2} which is defined as #define str2 "there". Now I want to concatenate them together into one string. So I use "$str1 ${str2}". But it turns out to be "hi ${str2}". How'd I do that? Thanks.

!define not #define, this is not C 😉

Section
Var /Global str1
StrCpy $str1 "hi"
!define str2 "there"
MessageBox mb_ok "$str1 ${str2}"
SectionEnd