It does not look like either the Client or workflow custom stamp rendering handles token formatting.

When a custom stamp is applied, the application that is applying it must take the custom text and generate a black and white bmp file that is then used to create the stamp.  When the token has formatting, the token is not converted, but the literal token text is inserted instead of the formatted token value.

There are only 2 ways I can see getting the date formatting as you want it.  First is to set the regional settings on the Workflow server to have that date format (this would effect the date format for all thing workflow does) or the second is to use a custom script to generate the bmp, the stamp from the bmp, and then apply the stamp. 

You can see Code to create and apply stamp in Workflow Apply Stamp Limitations

So how do I access workflow server to make changes?

I did not notice that you had marked this as Cloud.  There is only one way to do it in Cloud and it is not for the faint of heart.  This would require scripting and in cloud that can only be done by running a local Remote Agent in your office that is connected to the Cloud workflow.  Then you would have to create a DLL file that does what you want to do and configure that in your remote worker.  Then finally, you would have to call the remote agent DLL through your workflow.

Due to the fact that this process must create a one time use stamp and apply it to the document, you would need to create a licensed user in Cloud that then the DLL can use to log into the repository, create the stamp, and apply it to the document.

So while it is possible to do, it is not something to easily accomplish if you do not have a local server to run the remote agent and/or someone proficient at coding.