How can I setblock of a button that is always pushed?

You should be able to use the Debug Stick to alter the "state" of the button to be "powered". You can give yourself a Debug Stick with /give @s debug_stick. To learn more about block states, this page can help you https://minecraft.fandom.com/wiki/Block_states, and for the debug stick, this page: https://minecraft.fandom.com/wiki/Debug_Stick.

Note that if the button is right clicked after you alter the block state with the Debug Stick it will go back to being unpowered, if I remember correctly.