Using Placeholder in Concat() Function used in an Expression

[shannoypaul]

In the following expression $1 is used as a placeholder, $1 doesn't work within the concat() function

If(GetSelectedCount($1)=0,Null(),'&select=' & '$1,' & Concat(distinct$1,','))

Note: This is a definition to a variable and the variable is parameterized

TIA,

Shannoy