Probably. Depends how the phone is connected to Asterisk and what the flash button does.
SIP phones don’t really have flash buttons, and if you have a SIP phone claiming to have one, you will need to read the phone’s manual, as you need to configure the phone, rather than Asterisk.
If you have an analogue phone, the configuration goes into features.conf. You can also use this with SIP phones, but not using a “flash” button.
Analog phones have Flash or R button but how to use them I have no idea.
ATA Adapter is D-Link DVG-2032S. D-link have section where is flash button function but I guess that function works only in same device but I have 3 of them. I need that it will work through Asterisk…