Looking for a simple ScriptRunner Behaviour to make this happen on issue creation. It seems that I'm getting closer, but it's not working. The custom field name is "Request Type" and if the value is "Release Related" it should make the Fix Version field required.
def field1 = getFieldByName("Fix Version/s")
def field2 = getFieldById("customfield_13590")
if(field2.getValue().equals("15121"))
{ field1.setRequired(true) }
else { field1.setRequired(false) }