Why are they opening the portal?
We don't know their exact motives yet but if you have seen the end credit scene then remember they are not done with the Upside down:
In a post-credit scene, at a Russian base in Kamchatka, guards are referring to an unseen American prisoner as another prisoner is fed to a captive Demogorgon. - Wikipedia.
So we need to wait for the fourth season for an exact answer and we can only speculate at this moment. Most speculated one is they need a weapon against America or something but let's wait and see.
Why are they broadcasting the message (for whom?
It was a coded msg for there associate to know about the upcoming shipment, they can't use the phone as it can be way risky and they can't say it directly so coded msg. popsugar did a nice breakdown of the message. And if you remember then that shipment was green liquid to run the machine.