SharePoint List Custom Field Help Please!

Hi,

I'm trying to create a custom field in my SharePoint which is a combination of of a number of other fields within the SharePoint list. 

The URN is made up of [First letter of each word which makes up the Directorate] / [First letter of each word which makes up Team name] / Current year in YY format / [Incremental number].  Directorate and Team are existing fields.

For example, the URN for 

Directorate: Asset and Portfolio Development

Team: Asset Management

Year: 2021

Project # 1ST Project of the year 

Would have a URN of: AaPD/AM/21/001.

If I could keep the slashes between the individual elements which make up the URN that would be great.

Thanks in advance for anyone who can assist,