The Stryd platform will calculate your critical power automatically based on your running data. Critical power is the power you can hold for about 50 minutes (~10k distance for most).
Auto-CP is recalculated each time a run is uploaded into our data store. Your CP will only increase if you run at a higher power level for a certain time than you did in the last three months for the same time interval. After three months, your runs are no longer considered when calculating CP.
For example, I ran a 5k race in 20 minutes on the 1st of October. Any 20-minute effort after my race will likely be ignored until January since I am unlikely to run at race power in my regular training.
If you need additional assistance, contact firstname.lastname@example.org.