If you require Proxy Support for your installation of GameMaker Studio 2, this can be added simply by clicking the Proxy Settings button on the login dialog. GMS2 will then pass these settings to your proxy server, which should fix issues with not being able to license or download your runtimes.
It is worth noting that if you have set a proxy in your Windows Internet Options or in your Mac System Properties and it doesn't require a username/password, then it should just work "as is" and you probably will not be required to set up the proxy in GameMaker Studio 2.
Please note also that your proxy must allow the downloading of files by GMS2 - if it doesn't, then licensing or runtime downloads will continue to fail.
To add your proxy details, click this button on the login dialog:
Which will bring up the following window:
You can now add in the details required by your proxy. Some of the fields shown above are optional and if there is one you don't require to be filled-in, then simply leave the field blank.
The "proxy" field should be the IP address or name for the machine, plus the port to use, i.e.: xx.xx.xx.xx:port
If your proxy does require a username and password, then you will need to add them, plus the domain.
Once you have added the relevant information, click on OK and then add your YoYo Account details on the main login dialog and log in to GameMaker Studio 2 as normal.