I have added below 2 custom fields in Bug work item on Azure DevOps cloud
Is it possible to auto populate Release Environment when I select Release Number.
Is it possible to auto populate field based on other field selection in Azure DevOps?
The answer is yes.
You could add custom rules to the Bug work item on Azure DevOps, which allowing you to go beyond setting a default value for a field or make it required. Rules allow you to clear the value of a field, copy a value into a field, and apply values based on dependencies between different fields' values.
Following is my test custom rules:
Then, create other custom rules for other ReleaseNumber.
You can check the document Add a rule to a work item type (Inheritance process) for some more details.
Hope this helps.