Hi,
I have found an solution for RHEL 8 RHUI repo myself.
You can just follow the below to get RHUI for RHEL8.
1) Connect to your custom RHEL8 image.
2) Create config file with the following content.
vi rhel8.config
[rhui-microsoft-azure-rhel7]
name=Microsoft Azure RPMs for Red Hat Enterprise Linux 7
baseurl=https://rhui-1.microsoft.com/pulp/repos/microsoft-azure-rhel8
https://rhui-2.microsoft.com/pulp/repos/microsoft-azure-rhel8
https://rhui-3.microsoft.com/pulp/repos/microsoft-azure-rhel8
enabled=1
gpgcheck=1
gpgkey=https://rhelimage.blob.core.windows.net/repositories/RPM-GPG-KEY-microsoft-azure-release
sslverify=1
3) Save the file and run the following to install RHUI for RHEL8.
yum --config rhel8.config install 'rhui-azure-rhel8'
4) This will install RHUI for RHEL8
5) yum update