I am Not Sure What Your Intent is   

if you mean you have to add a function and call it before  main after the source is compiled   

then yes it is definitely possible   
all you have to do is run through some hoops like finding a code cave , assembling the new code in code cave , and detouring the first instruction of main to call the your function and return back to mains next instruction 

if you mean i want the function to be sequential then it might be possible if you can destroy and recreate the assembly inplace (kinda tough )

and no there is no label you need to calculate relative address / absolute address of the newly added function and use one form of e8 or ff25 call  

having source can help you in understanding the assembly but you have to play in assembly only if you have to patch a compiled executable  

a program prior to addition of encrypt proc will look like this (a simple msgbox in windows notice both caption and text are pointing to a string which is unintelligible)

[![enter image description here][1]][1]

you may need to implement the commented out src code inline for decrypting 

    .386
    .model flat, stdcall
    option casemap:none
    include \masm32\include\windows.inc
    include \masm32\include\kernel32.inc
    includelib kernel32.lib
    include \masm32\include\user32.inc
    includelib user32.lib
    
    .data
    Encrypted   db 217,243,234,245,252,249,255,254,183,227,176,228,229
                db 228,255,226,249,241,252,176,254,255,190,162,144
    
    .code
    start:
        ;call decrypt
    	call hello
    	invoke ExitProcess,NULL
        
    ; decrypt proc
        ; xor ecx,ecx
        ; complete:
        ; lea esi , Encrypted 
        ; add esi,ecx
        ; movzx eax , byte ptr ds:[esi]
        ; xor eax ,090h
        ; mov byte ptr ds:[Encrypted + ecx], al
        ; add ecx,1
        ; cmp byte ptr ds:[Encrypted + ecx - 1 ],0
        ; jne complete
        ; ret
    ; decrypt endp
        
        
    hello proc
        invoke MessageBox, NULL,addr Encrypted, addr Encrypted, MB_OK
        ret
    hello endp 
    
    end start



 


  [1]: https://i.sstatic.net/it6Kp.jpg