Create a custom Sub domain for API Gateway from AWS Console.
Step 1: Open AWS Managed Console (aws.amazon.com)
Logging into your account.
Step 2: Navigate to API Gateway from services menu
Step 3: Go to Custom domain names > create
Step 4: update the details and click on create domain name
Step5: Select the custom domain name you have created > Click on API Mapping to map our existing API to custom domain name.
Step 6: Click on API Mapping
Step 7: Click on Add New mapping
Step 8: Select API Gateway and Stage from dropdown and click on save.
Now you are good to go with your custom subdomain name.
Please provide your valuable feedback, and please do let me know if you still facing any issue.