Fill GOHEAD structure from a field symbol

Hi all , i need some help with a user exit i'm working on .

I need to validate a value in MIGO but  the structure where the value is the GOHEAD , i read in other forum that using field symbols help , and it was right , i read the value form the structure but now i need to modify the value in that structure but don't know how , i hope can help me , this is the code i use for now

include ZXMBCTOP

FIELD-SYMBOLS:  <fs1> TYPE ANY,

                 <fs2> TYPE ANY.

include ZXMBCU02

DATA : XABLN LIKE GOHEAD-XABLN.

       ASSIGN '(SAPLMIGO)GOHEAD-XABLN' TO <fs1>.

       ASSIGN (<fs1>) TO <fs2>.

       XABLN = <fs2>. " this code enable you to retrieve the delivery note entered

       IF XABLN EQ SPACE.

         XABLN = 'VACIO'.

       ENDIF.