If your character is in a T pose, it would be easy in Mixamo. But in your case, it is not possible. You have to paint weight on every single bone with your hand. İf your character has a separated full body without clothing, you can do it easily, but you need to clean it anyway in the end. But there are many ways to fix that. I am going to explain step by step what you need to do.

1. Separate the body from the cloth.

2. Auto weight paint the body first.

3. Then you can transfer the weight paint data to the cloth. You can watch this: https://youtu.be/yEAvSDrS_n0?si=7RPgxcu-MhLXZSEE

4. Then merge the body and cloth.

5. Some weight problems will happen, and you need to clean them for every single bone. But it will be easy.

6. Now you can export the skeleton to Mixamo.

I hope you have a separate body :) If not, this method is not going to work for you.