You can’t add custom DNS records type to Admin Center
This normally happened if you have only PartialRedelegation upon adding your domain. We need to change this to FullRedelegation especially if the NameServer is pointed to Microsoft 365 so that we can add other records like records for your website.
Go to M365 portal > Settings > Domains.
Select the domain and click manage DNS.
Click on more option and choose “Set up my online services for me”.
Click continue until you reach to this page.
You can now add the IP address of the website during the set up.
Complete the set up and go back to manage the domain to see if you have a capability to add custom DNS types