How to add a values (html select) to custom field?
You can use managed hook to create custom field on install


Create Option Group
2.Create option values for option group
Create custom group
Create custom field.


There is another way of doing this i.e by creating xml file. A good example to refer would be https://github.com/compucorp/civihr/blob/master/hrcareer/xml/auto_install.xml

HTH

Pradeep