Hi Sama,
Thank you for reaching out. In Adwords, if you would like to add YouTube video to UAC campaign, you need to manually set the campaign's YouTube Video assets through the Google Ads UI. If you already have YouTube Video assets in the Google Ads UI, you will be able use the MediaService.get() to retrieve the MediaId of the youtubeMedia. Then you could use the CampaignService.mutate() to create a UAC campaign with the setting as UniversalAppCampaignSetting including the youtubeVideoMediaIds. Please let me know if you have any further questions.
Thanks and regards,
Xiaoming, Google Ads API Team