/A is reserved VFP command-line switch. See http://berezniker.com/content/pages/visual-foxpro/how-pass-parameters-vfp-exe for details.
Removing '/' should fix the problem

ImsBackup.exe AutoStart AutoClose

>I need to pass two parameters to my application using this format from DOS prompt:
>
>

>ImsBackup.exe /AutoStart /AutoClose
>

>I'm using this in my main app:
>
>

>PARAMETER l_Param1,l_Param2,l_Param3,l_Param4,l_Param5,l_Param6
>
>ACTIVATE SCREEN _Screen
>=MESSAGEBOX(l_Param1 + l_Param2)
>

>
>The messagebox with show
>
>

>AutoStart /AutoClose
>

>
>But I’m losing the 1st letter of the 1st parameter. What could be causing this?