If you are wanting to see this in a column in a project list / report, you would use the following text mode. You can rename the line for displayname= to whatever you want your column to be called.
displayname=Reference Number - Planned Start Date (Year)
textmode=true
valueexpression=CONCAT({referenceNumber}," - ",RIGHT(YEAR({plannedStartDate}),2))
valueformat=HTML
If you wanted to create this as a calculated custom field, it would look like the following on your project custom form (text format)
CONCAT({referenceNumber}," - ",RIGHT(YEAR({plannedStartDate}),2))
More information on calculated data expressions here.
Hope that helps!