Some of the DNS hosting servers do add the domain value automatically at the end of the hostname.
For example, if you wanted to add a CNAME value as following:
Depending on how your DNS server hosting behaves you may or may not need to add the 'massmailer.io' for the HOST.
If your DNS host adds the domain automatically at the end of the HOST then you will just have to add as follows:
Some DNS hosts may want you to add period, "." at the end as shown in the table below:
So, we recommend that you work with your DNS host support team to make sure that the values have been added properly.
You can check the DNS values if they are added properly or not at https://mxtoolbox.com/NetworkTools.aspx
Following is an example setting on hostlabs.com