I did this small example for another context but its useful code to know. Because the "invoke" call is a MASM macro it can directly use the address from GetProcAddress.
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include \masm32\include64\masm64rt.inc
.code
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entry_point proc
LOCAL hLib :QWORD ; loal handle for DLL
LOCAL mbox :QWORD ; local handle for procedure
mov hLib, rv(LoadLibrary,"user32.dll") ; load the DLL
test rax, rax ; test if it exists
jnz @F
.exit ; exit on zero if it does not
@@:
mov mbox, rv(GetProcAddress,hLib,"MessageBoxA") ; get the procedure address
test rax, rax ; test if it exists
jnz @F
.exit ; exit on zero if it does not
@@:
; ------------------
; call the procedure
; ------------------
invoke mbox,0,"Win64 : LoadLibrary : GetProcAddress", \
" Isn't MASM Beautiful",MB_OK
invoke FreeLibrary,hLib ; release the library
.exit
entry_point endp
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end