fastcall how to use for more than 4 parameters

Question

I was trying to build a function in assebmly(FASM) that used more than 4 parameters. in x86 it works fine but I know in x64 with fastcall you have to spill the parameters into the shadow space in the order of rcx,rdx,r8,r9 I read that for 5 and etc you have to pass them onto the stack, but I don't know how to do this. this is what I tried but it keeps saying invalid operand. I know that the first 4 parameters I am doing right because I have made x64 functions before but it is the last 3 I don't know how to spill

proc substr,inputstring,outputstring,buffer1,buffer2,buffer3,startposition,length
;spill
mov [inputstring],rcx
mov [outputstring],rdx
mov [buffer1],r8
mov [buffer2],r9
mov [buffer3],[rsp+8*4]
mov [startposition],[rsp+8*5]
mov [length],[rsp+8*6]

if I try

mov [buffer3],rsp+8*4

it says extra characters on the line. I also saw that somepeople use rsp+20h, rsp+28h etc but that does not work either. how do I call more than 4 parameters using fastcall on x64? also do I have to make room on the stack? I saw some people have to put add rsp,20h right before their spill code. I tried that and it did not help the invlaid operand. thanks

update

after playing around with it for a little bit I found that the only way it seems to work is if I spill the first 4 parameters and then ignore the rest 5-infinity

proc substr,inputstring,outputstring,buffer1,buffer2,buffer3,startposition,length
;spill
mov [inputstring],rcx
mov [outputstring],rdx
mov [buffer1],r8
mov [buffer2],r9

;start the regular code. ignore spilling buffer3,startposition and length 

Answer 1

On x86/x64-CPUs this following instructions does not exist:

mov [buffer3],[rsp+8*4]
mov [startposition],[rsp+8*5]
mov [length],[rsp+8*6]

Workaround with using the rax-register for to read and for to write a values from and to a memory loaction:

mov rax,[rsp+8*4]
mov [buffer3],rax
mov rax,[rsp+8*5]
mov [startposition],rax
mov rax,[rsp+8*6]
mov [length],rax