first step is to identify the name of your wireless network interface. To do so execute:

$ ls /sys/class/net
enp0s25  lo  wlp3s0

Depending on your Ubuntu 20.04 system the wireless network interface name would be something like: wlan0 or like in this case it is wlp3s0.

sudo vim /etc/netplan/50-cloud-init.yaml

# This file is generated from information provided by the datasource.  Changes
# to it will not persist across an instance reboot.  To disable cloud-init‘s
# network configuration capabilities, write a file
# /etc/cloud/cloud.cfg.d/99-disable-network-config.cfg with the following:
# network: {config: disabled}
network:
    ethernets:
        eth0:
            dhcp4: true
            optional: true
    version: 2
    wifis:
        wlp3s0:
            optional: true
            access-points:
                "SSID-NAME-HERE":
                    password: "PASSWORD-HERE"
            dhcp4: true

sudo netplan generate

sudo netplan apply

ubuntu connect wifi on startup