Try this variant. What I am testing is if exist is being handled differently.
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include \masm64\include64\masm64rt.inc
.code
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entry_point proc
LOCAL clp :QWORD ; command line pointer
LOCAL rvl :QWORD
call GetCommandLine
mov clp, rax
rcall MessageBox,0,clp," GetCommandLine",MB_OK ; show raw command line
mov clp, rvcall(cmd_tail)
rcall MessageBox,0,clp," cmd_tail",MB_OK ; get the command tail
.if len(clp) == 0
rcall MessageBox,0,"Nope"," cmd_tail",MB_OK
.else
rcall MessageBox,0,"Yurp"," cmd_tail",MB_OK
.endif
.exit
entry_point endp
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end