You want to let the user start a GuiXT InputScript by clicking a button in Excel

Assign a macro to the button that starts "guixt.exe" with the option "input=..."

We assume that a SAP GUI session is already open for the system in which you want to start the InputScript.

Sub upload_items()

  Call Shell("C:\Program Files (x86)\SAP\Frontend\sapgui\guixt.exe" & _
                     "input=OK:process=C:\guixt\excel_upload_items.txt", vbHide)
End Sub

Starting a series of InputScripts

If you want to start a series of InputScripts with the button click, for example one InputScript call for each Excel row, you need to synchronize the InputScript execution in your Excel macro. Otherwise a running InputScript could be interrupted by the next one and deadlock situtations may occur.
A possible approach for synchonization is to use an empty status file that is written when the InputScript has terminated. In the Excel macro you wait until you detect the status file:
' Wait for Status file to confirm that the InputScript has finished
   Application.Wait Now + TimeSerial(0, 0, 0.01)
   FindIt = Dir("C:\Guixt\excel_status_file.txt")
Loop While FindIt = ""
' Now we can start the next InputScript

At the end of the InputScript, set a GuiXT status variable

Set V[excel_status_file]   "C:\Guixt\excel_status_file.txt"
In the GuiXT script that follows your InputScript you query the status variable, create the file and clear the status variable:

if V[excel_status_file]
  OpenFile "&V[excel_status_file]"   -output
  CloseFile "&V[cexcel_status_file]"  
  Clear V[create_empty_file]
This way your are sure that the InputScript has ended when you write the status file.