I agree with @speed57, it sounds like he is wanting to set up an auto mode, and a manual mode. So when set to auto mode the value would be a preset value (possibly hard written into code), when in manual mode the value can be set via the slider.
This is definitely possible, and probably not too hard. Check out this thread:
https://community.blynk.cc/t/arduino-temperature-personalization/13449
They are talking about setting a value via a slider. You will probably need to do this first.
Once you can set the value via a slider, a simple IF statement to check if the button is set to manual or auto and there you go.