Embed XMP metadata In Output File options are disabled for MP4

I had the same problem with Media Encoder... but used Premier Pro Export instead.
Scenario:
I wanted to upload new videos to my Youtube channel as 24FPS or 30FPS with codec H.264 .mp4 (4K UHD 2880x2160p and 2K 2560x1440p, Max Render Quality and Max Bit Depth, Vbr2 pass and wanted to export using video render previews to expediate the render process)... there is No Metadata associated with these new projects... ( if I import a source video that has metadata... i can view the metadata by selecting the "Metadata " option under the windows tab in PremPro).
In the Premier Pro Export section i used Match Source (bitrate, same FPS as source video).
At the bottom selecting the Metadata dialog... opens the Metadata selections... and its default is "All Files".
I selected "Dublin Core, Media Management, Rights management, IPTC properties to input Titles, Descriptions, keywords, Copyright notice etc"... only the ones that i really need for the video.
I saved it as a Metadata preset for e.g.  2K or 4K 24FPS H264 METADATA. Doing so gives me the option of what Metadata presets that I want to add to different videos... instead of having the "All Files" metadata added to my videos.
I Exported the new video from Premier Pro with the new metadata preset. To check if metadata was successfully embedded into the final exported .mp4 file... i restarted PremPro... new project and just imported the new video that i just exported. I created a "new Sequence" from the imported new file... now it shows in the timeline.
Clicking the windows tab and selecting the Metadata option... shows the successful embedded Metadata that i wanted to make sure was embedded into the new video before uploading to YouTube... "happy Dance" 🙂
Ok... now I want to use Media Encoder to do the same thing.
Using the same PremPro Export settings... then "Send to Media Encoder"... waiting, waiting, waiting for ME to load...
i created a Preset in ME... same as what i have in PremPro ( e.g.  4K 24FPS H264 METADATA).
Problem that i have in ME is that in the Metadata dialog section, i cannot select my preset... and even if i could the Preset doesn't have the Metadata fields filled in ?? ... so just for now i just use PremPro export in which exports h.264 .mp4 files with Metadata.
i'm currently Googling "how To import Premier Pro Metadata xml files into Media Encoder Metadata presets"... but no luck yet (Dec 2024)... will post edit/update later if i find out. Have a Great Day! Hope that helps! 😉
P.S.
Just for the heads up... if you have a video source file that already has metadata associated with it (from a client or something) and have different Metadata dialog setup in PremPro export section... and then send to Media Encoder to render... you may get an error report that says "mismatched metadata" ... or "multiple selections" or something... so you would have to have same Metadata info in the source file and the PremPro export Metadata dialog file.... and/or deselect "Clip linking with Metadata" in preferences or in the Metadata window... (honestly, don't quote me on that... just providing areas on where to look if  you have that promblemo)!!

If you use H264, it will export an MP4 and this disables the metadata embed option. If you go back and set the export to be Quicktime, the embed option is enabled. So I think this means you can't use ME to embed metadata into an MP4 but you can for an MOV. This is unfortunate b/c I have the same need: to add metadata to an MP4.