That code already has the feature to use the ability with IN_USE button:
Code:
public fw_PlayerPreThink(id)
{
if (!is_user_alive(id))
return FMRES_IGNORED
new button = get_user_button(id)
new oldbutton = get_user_oldbutton(id)
if (g_bind_use[id] && is_user_zombie(id) && get_user_class(id) == g_class)
{
if (!(oldbutton & IN_USE) && (button & IN_USE))
drag_start(id)
if ((oldbutton & IN_USE) && !(button & IN_USE))
drag_end(id)
}
if (!g_drag_i[id]) {
g_unable2move[id] = false
}
if (g_unable2move[id] && get_pcvar_num(cvar_unb2move) > 0)
{
set_pev(id, pev_maxspeed, 1.0)
}
return PLUGIN_CONTINUE
}