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?