Connect to wifi before Lubuntu install (prepare in live mode?)

I am in the situation where only bootable external media I can use is an USB flash and only option I can use to connect to Internet is Wi-Fi.

I am trying to install Lubuntu on an older Acer laptop and I am having the issue with the grub installation, I've read that it can be solved by connecting the device to the Internet (people are also advising to select .../sdX instead of .../sda, but the installation is stuck there, so I am trying this way).

Also I would like for it to get any updates while installing, that's reason No.2.

So how would I connect to wifi, before the installation? Can it somehow be prepared from the live run?

Lubuntu ISO I am using: lubuntu-16.10-desktop-amd64.iso

I will be grateful for any advice/help ! Thanks :)
(I am a linux sometimes-user, I know a few basic commands, but I am not an expert...)