How do I backup my iPhone when the front screen is damaged and it's asking me to enter my passcode and trust computer? its the same computer I plug it into all the time with the same Apple ID as my iPhone

Hard to say why it’s asking for a passcode, more than likely a security related thing if it’s been over 24+ hours since the phone was last unlocked.

Unfortunately, you can’t bypass that unless you can put the code in, so screen repair would be needed unless you have a recent backup from before the screen cracked.

Even if the Mac is logged in with the same Apple ID you need to enter the passcode. So you can't take a backup.