Wait patiently is the only answer. Android 12 Beta program is going on in Google Pixel phones and other brands, not for Samsung. Samsung will give Android 11 One UI 4.0 Beta for the latest flagship S series phones after the launch of the stable Android 12 update. After that, the stable update of Android 12 One UI 4.0 will be launched for all the Samsung phones that are eligible for it. The official list of eligible phones are not available now.