I’ve added a new DNS domain, which is ’ mydomainname.io ’ without ‘www’, and in this DNS domain, I have made two things

1. add a 'A record’, which is : @ , ip address.
2. add a 'CName record’, which is : * , mydomainname.io.

when everything is complete, I can go to the mydomainname.io however can’t reach to www.mydomainname.io.

I followed some previously solutions like this one https://www.digitalocean.com/community/questions/domain-with-and-without-www,
say in the CName record:

Enter Name: www
Enter Hostname: @

but when I did this, the result turned to be
www , mydomainname.io.

and nothing changes afterwards, I still can’t reach to www.mydomainname.io