How to create a validation in VBS if a certain supplier was selected then another field is mandatory

Summary:

I would like a rule/validation if a certain supplier was selected then then one of our tax fields would become a certain selection.

Are there any instructions on how to create common validations?

Content (please ensure you mask any confidential information):

We would like if in this scenario, 'RBS' is selected as the supplier, then 'Out Of Scope' to be selected. I can't see how to do so in Conditions or Validations or if that even is possible.

Many thanks

Version (include the version you are using, if applicable):

25A

Code Snippet (add any code snippets that support your topic, if applicable):