Use the {app}
constant. The reference describes it as:
The application directory, which the user selects on the Select
Destination Location page of the wizard. For example: If you used
{app}\MYPROG.EXE
on an entry and the user selected "C:\MYPROG"
as the
application directory, Setup will translate it to
"C:\MYPROG\MYPROG.EXE"
.
Optionally you can use the WizardDirValue
function. This one is described as:
Returns the current contents of the edit control on the Select
Destination Location page of the wizard.
Unlike ExpandConstant('{app}')
, this function will not fail if called
after the wizard is shown but prior to the user selecting a directory.
Rather, it will return the default directory name.