In Maya, import your auto-rigged character with the latest version of the FBX plug-in. Make sure the fbxmaya.mll plug-in is loaded: Window > Settings/Preferences > Plug-in Manager.
Run the Mixamo Auto-Control Setup python script (make sure it's in a Python tab). You may want to make it a shelf button for easy access in the future.
Press the “Rig Character” button. You now have a fully animatable rig of your character. There are display layers and selection sets for your convenience.
Mixamo on Maya FBIK
Export FK skeleton from Maya FBIK. Select the FBIK FK skeleton (starts with node hipsFK), make a copy, rename the root joint to the same as the original (hipsFK), and export the newly created FK skeleton hierarchy in FBX format (character in t-pose, default export settings of FBX plugin, Y-up). IMPORTANT: Mixamo only needs the FK joints of your rig. Make sure your character is in t-pose on frame 0 of the scene.
Select a motion model, Mixamo has hundreds of online motions, Many more being added each week.
Design & Synthesize Motion using built in sliders and controls.
Upload your FBX model. If your character is rigged using Maya FBIK or its naming convention it will map automatically.
Final Preview & Download. Preview to your heart's content, only download when you're satisfied in FBX format.
Import the downloaded FBX motion into Maya. File > Import and Select "Update Scene" in the FBX import plugin and check the "Fill Timeline" to update the timeline of your scene. Select all the joints of the updated FK skeleton and select all the imported keys. Select the corresponding FK joints in the original FBIK rig and paste the keys.