Thank you Simorq.
I don't know where I was with my head....
It was supposed to look like this.
Code:
WizardForm.DirEdit.Text := RemoveBackslashUnlessRoot(ExpandConstant(Trim(GetValStrEx('InstallOptions', 'DefaultInstallDir', '')))) + GetAppName('')
WizardForm.DirEdit.Text := AddBackslash(ExpandConstant(Trim(GetValStrEx('InstallOptions', 'DefaultInstallDir', '')))) + GetAppName('')