What version and edition of InstallShield are you using? Also what project type are you using? (InstallScript, InstallScript MSI, Basic MSI? )
Assuming Basic MSI, InstallShield has a built-in pattern to support this story:
1) Click on the Project Assistant Tab
2) Click on the Installation Interview Icon (Page)
3) Click Yes for "Do you want to give the users the option to launch your application when the installation completes?
4) Click the browse button to select the EXE that should be the target of the operation.
By default the checkbox (launch program) on the setup complete dialog will not be selected. To select it automatically add the property LAUNCHPROGRAM to the property table and set it to a value of '1'.