Find the download URL for the latest binary from the official Downloads page.
At the time of writing, this is v0.12.6, which was released on June 11, 2020.
Download the binary, and then install the package and its dependencies. For example:
wget https://github.com/wkhtmltopdf/packaging/releases/download/0.12.6.1-2/wkhtmltox_0.12.6.1-2.jammy_amd64.deb
sudo dpkg -i wkhtmltox_0.12.6.1-2.jammy_amd64.deb