With consistent training, your Lactate Threshold Heart Rate for cycling will remain particularly stable over several years. However, you will be presented with occasions when your LTHR appears unusually high (due to dehydration or recent inactivity for example) or unusually low (due to heavy fatigue for example).
For this reason, I suggest that you don't use a specific assessment to determine your LTHR, but instead, look at reliable historical data (up to ~3 years ago) to establish an appropriate heart rate range. When you have been cycling at threshold (FTP) intensity for an extensive period, or during shorter threshold intervals that were long enough, or frequent enough, to illicit a steady state heart rate - what narrow (~5 bpm) range do you typically see when your heart rate is in a steady state?
If you have enough historical data to look back upon, you will certainly find a narrow range that frequently occurs. If you need a specific value (to input into TrainingPeaks for example), then simply use the heart rate that falls within the middle of this narrow range. If you are a beginner athlete who doesn't yet have enough data to determine a LTHR range, but only have heart rate (and RPE) as a means of measuring intensity - then you may wish to perform an initial assessment to get a baseline value to work from.
In this instance, I would recommend that you perform a 30 minute maximum effort time-trial over a constant grade, with no interruptions or coasting opportunities. Provided there were no factors that would significantly affect the validity of your heart rate data from this assessment, your average heart rate from the final 20 minutes of this 30 minute assessment - should be a reasonable measure of your LTHR.
Please note, in my experience, wrist based heart rate monitors are not yet consistently accurate enough to be used as a means of measuring intensity on a regular basis. Occasionally, during low intensity exercise, they can seem reasonably accurate. However, far too often they are failing to respond quickly enough (or at all) to increases in intensity that are easily detected by chest strap based heat rate measurements. For this reason, if you are looking to use heart rate as a metric in training, you should be using a reliable chest strap based monitor.