Context:

When registering the HCP Domain in a customer tenant > Settings > Domains the "_" sign has been used like: my_domain.eu.eopchp.com

Issue:

When accessing the tenant though a web browser, it is not possible to login.

Resolution:

When registering the DNS name for your HCP domain, please be advised that the name must comply in accordance with RFC 1035.

Only allows letters, digits and hyphens are permitted. In case a composed name is to be used, it can be entered like my-domain.eu.eophcp.com

Ref: 

https://datatracker.ietf.org/doc/html/rfc1035 

<letter> ::= any one of the 52 alphabetic characters A through Z in
upper case and a through z in lower case

<digit> ::= any one of the ten digits 0 through 9

Note that while upper and lower case letters are allowed in domain
names, no significance is attached to the case.  That is, two names with
the same spelling but different case are to be treated as if identical.

The labels must follow the rules for ARPANET host names.  They must
start with a letter, end with a letter or digit, and have as interior
characters only letters, digits, and hyphen.  There are also some
restrictions on the length.  Labels must be 63 characters or less.

For example, the following strings identify hosts in the Internet:

A.ISI.EDU XX.LCS.MIT.EDU SRI-NIC.ARPA