Reference number

Hi

I'm trying to translate a Japanese 'completion of certificate' for trainees.

There is a reference number at the header.
The reference number is just for general number reference, not the quoting text kind of reference.

My question is, when we put reference number (ie, Ref12345), should we put ':' next to the `ref`?

-->
Ref:12345

and

Ref12345

which one's correct?

Thanks

Better to put Ref. No: 12345