I suppose you are in the l3 interface for the ip-helper command? If so, that command should work.
just to be sure, example:
config t
interface vlan x
ip helper-address x.x.x.x
https://gtacknowledge.extremenetworks.com/articles/How_To/How-to-change-the-IP-Helper-address-on-K-Series/?q=ip+helper&l=en_US&c=Extreme_Software%3AModular_N_S_K_7K&fs=Search&pn=1