Follow below mentioned steps to easily create a sub-domain -
Step 1 : Login to your app account and click on dropdown menu besides profile icon
Step 2 : Select My Domains from the dropdown menu
Step 3 : Open the three-dot dropdown menu as highlighted in the screenshot
Step 4 : Select Manage DNS Record from the dropdown menu
Step 5 : Click to open Type field dropdown menu
Step 6 : Select CNAME or A here, and Complete the required fields :
Step 7 : Once done, click on Save
Step 8 : You can see the new record on the Manage DNS Record screen