Xbox App - Microphone doesn't work when I'm streaming

What specifically doesn't work? The stream audio or the party audio?

Was that not covered in the OPs post?

"I'm unable to use the microphone on my Steelseries when coneected to my PC using a Remote Streaming session"

"just not when joining a party during a stream."

Seems to me that the OP is playing using the Remote Play function but the Microphone doesn't work when in a party chat whilst using the Remote Play. the Mic works fine when directly connected to the Xbox or used via the Xbox App

I have done this and it doesn't work, I'm using a separate microphone but every time I open an xbox app it refuses to use it

No, it doesn't work when I plug it into the console. It doesn't recognize it so it doesn't let me assign it to a profile.

I heard that you could use both your headphones and microphone when streaming if you wire it to a PC. Without it, only my headphones work.

That generic support doesn't really apply here. The problem is with a PC microphone, not an Xbox One microphone. The Xbox App on PC detects the mic and it works if I start a party via the app, but it doesn't work in any capacity on the Xbox One for parties
or game chat.

There are USB microphones that will work with the XBOX. One that works and is made for XBOX is the Turtle Beach Stream Mic. I personally have one and works great. Before purchasing it, I tried a BLUE Yet microphone which is USB and did not work. I
am not aware if there are others that work but the Stream Mic works. It also has a PC app for some fine tuning and a headphone jack for Mic monitoring and game audio