The regular trend has no such customization, current value can only be included as a column in the table below the trend.
If you build a PG2 display, you can put a embedded minitrend primitive there along with a numeric value for Current Value; but then you have a PG2 display, not a trend... the minitrend is hardcoded for the very unfavorable Momentary Treatment. Poor performance. No secondary log data possible. Very short time span possible if primary log is fast. Only 86400 values can be drawn (=1 day of 1s samples).
You can change the column visible in trend template which is used for the particular trend. There are many column fields are available like current value, low range , high range etc.. You can find the trend templates in Library structure.