defining screen fields using FIELD-SYMBOL

Hi..

I need to update a user field in co01. I ws able to get the material i wanted to display on the user field through this code.

DATA : material like CAUFVD-matnr.

FIELD-SYMBOLS: <F> TYPE CAUFVD-matnr.

assign ('(SAPLCOVF)CAUFVD-matnr') to <F>.

If SY-SUBRC = 0.

material = <F>.

endif.

now i want to assign this to

(SAPLCOVF)AFVGD-USR00 ..

The exact opposite of wht i do in the above code. But the problem is how do u define a screen field in FIELD-SYMBOL.

i tried

FIELD-SYMBOLS: <F1> LIKE SAPLCOVF(AFVGD-SLWID).

But tht doesn't work?? Cn anyone help pls? Tnx

Keshi