Part of the candle, wick provides valuable information on immediate price in forex. It can be employed in multiple effective ways. During the last few years we have created hundreds of MT4 expert advisors employing the candle wick as a signal or filter. We have learned that some approaches work and some not. This is the shortlist of our preferred ways of using the candle wick as entry signal.
How to trade candle wick in forex
Look only on timeframes above 1 hour.
Lower the timeframe, more chaotic the price. In our experience, wick loses its edge on timeframes below 1H. Avoid them when considering it.
Always consider the trend/range.
The wick only gets its value, by considering it together with trend or range. As such it is valuable filter that can increase the accuracy of almost any entry signal.
What wick to look for in the trend

In trend, we will be looking for relatively narrow candle body with wide to ultra wide wick.The candle wick against the direction of the trend should be at least ⅓ larger than cadle body. The wick in the direction of trend should be relatively flat.

This candle and wick especially signalizes that trend countering move (test) has hit the support/resistance. This could be the ideal place to enter the trend. In most of the scenarios it is even more beneficial to wait for next (confirmation candle). The confirmation candle should surpass the high/low of previous candle in the direction of trend. Ideally, it will do so, with wide and strong momentum body.

More we increase the disproportion between wick and candle body (looking for ultra wide wick and flat candle). More accurate and less frequent the signal becomes.
What wick to look for in the range

In range we will be looking for signs of price hitting support/resistance at the boundaries of the range. This is signaled by wick briefly penetrating the S/R zone while failing to close outside the tunnel (tunnel might be represented by S/R, bollinger bands or similar tunnel indicator). The wick penetrating the S/R must be at least ⅓ larger than the candle body. This candle should be accompanied by high volumes (frenetic activity resulting in failure to penetrate the zone). We can enter immediately upon large wick candle close, or we can wait for confirmation candle. Confirmation candle should close in the direction of bounce. Usually it is not the best place to enter the trade as it might be too late to enter the trade on a good price.Therefore we prefer entering immediately after first candle close (signal wick) as it offers statistical advantage in better price.
Placing the stop loss

End of a wick is great place to place stop loss. Typically price will have some trouble reaching those values again. Fundamentally correct entry will typically not get stopped out with this SL placement. It can be used both in range and trend. Few pip gap can be added just to be sure we avoid any unnecessary stop outs.
Consider the few candles before the signal candles.

Meaning of wick can change based on few previous candles. Wick should be read in consideration of events on last few candles. For example:Strong bullish move (momentum) followed by wick signaling possible trend trend reversal (weak signal, avoid). Few flat candles followed by extreme wick, followed by strong candle into opposite direction (strong signal, enter).
Avoid formations with wick swinging wildly into both directions

Except few specialised systems looking exactly for this type of scenario (scalping MT4 Expert advisor). This is not the best place to consider or trade the candle wick. Market is too volatile, choppy, chaotic. The entry cost will be too high, making the effective entry difficult.
Look for two types of wick situations
Test in the trend (end of a correction). Wick at top/bottom of the trading range. Those two distinctive market situations are offering best conditions to employ and succeed using the candle wick as a signal or filter.