You can do it using Validation based on regular expression. Let's suppose the field you want to make mandatory is a Select List called "Colour". You can compose a parsed text with virtual field "Issue resolution" and custom field "Colour".
Then you validate the value for matching to a regular expression that describes the allowed values or for not matching to a regular expression that describes forbidden values.
In this particular case it's easier to use the second option. This is the configuration screen for our example:
Text to be parsed is:
%{00028}&%{10300}
Fixed&\s*
Once configured, the validator will look like this: