See below explanation
Explanation:
Add a data disk
Step 1. In the Azure portal, from the menu on the left, select Virtual machines.
Step 2. Select the Web01 virtual machine from the list.
Step 3. On the Virtual machine page, , in Essentials, select Disks.

Step 4. On the Disks page, select the Web01-Disk1 from the list of existing disks.
Step 5. In the Disks pane, click + Add data disk.
Step 6. Click the drop-down menu for Name to view a list of existing managed disks accessible to your Azure subscription. Select the managed disk Web01-Disk1 to attach:

References:
https://docs.microsoft.com/en-us/azure/virtual-machines/linux/attach-disk-portal