Based on GitHub documentation I can create a custom Bug Report template, and in that i can customize the Markup that fits the business need.
OOTB bug and custom templates have same fields (Assignees, Projects, Labels, Milestones... etc) But how can I add custom fields to it (For e.g. Priority, Severity, Planning Status etc...)
Another example is AzureDevOps allows definition of custom templates and custom fields for bugs or feature enhancements.
Can I achieve similar in GitHub templates ?
Apr. 2020: From the documentation, there does not seem to be any option to add custom fields.
That maybe because of the management involved with any new field: for instance, limiting the values of "Priority" to only a fixed set (like "low", "medium" and "high")
Since each custom field would require its own set of rules and validation, this is not currently available for GitHub issue template.
Note: the June 2021 "New beta features within GitHub Issues, with better ways to plan, track, and manage projects" (see GitHub Issues page or the FAQ) includes in its documentation the possibility to "create and customize multiple views by filtering, sorting, grouping your issues and pull requests, visualize work with configurable charts, and add custom fields to track metadata specific to your team".
But, as confirmed by AlexMakarov in the comments, it does not help to populate custom fields via issue template.