I have this Barlow which, combined with this t-ring adapter allows me to shoot with my Nikon at prime focus with an additional 2x magnification.
I think the way the magnification is going to work is you take the focal length of the telescope divided by the size of your camera's sensor, which is the magnification you'd get with just the camera and the telescope. So with that 500mm focal length scope and your camera's 35mm sensor (although I think the sensors aren't exactly 35mm) then you'd end up with 500/35 which is a 14x magnification. So with the 2x Barlow I linked you'd get 28x, although you'll also have to compensate with the slightly dimmer magnified image.
A couple of barlows have T-threads built in. Here's an example:
https://www.amazon.com/Celestron-Adapter-Barlow-Universal-T-Ring/dp/B00009X3UV
The threads at the top of that barlow would screw directly into a DSLR T-ring like this: https://www.amazon.com/Celestron-93402-T-Ring-Camera-Attachment/dp/B0002379UO/ (you would buy the right T-ring for your brand of camera).
Alternatively, you can just get a T-ring + 1.25" nosepiece like this: https://www.amazon.com/SVBONY-Compatible-Standard-Telescope-Microscope/dp/B00W123RBE/
And then a normal barlow like this: https://www.amazon.com/Celestron-93529-X-Cel-1-25-Inch-Barlow/dp/B005G4HT6I/
And then the camera + t-ring + 1.25" nosepiece will insert into the barlow just like an eyepiece would.
Either approach works. The benefit of the first approach is that the connection is a bit more stable, and it minimizes how much extra multiplication you get from the barlow since the distance to the sensor will be a bit shorter. The downside is that virtually all the barlows that feature that thread are cheap quality.
The benefit of the latter approach is that you have your choice of barlow, so you can get a higher quality one (or even a 2" barlow if you need a 2" camera adapter due to having a large sensor).
What are you trying to do here? Deep sky imaging? Planetary imaging? Full disk lunar imaging?
Do you actually need the extra image scale that a barlow can provide, or are you trying to use a barlow just to reach focus in your scope?