Building an expression in Azure Enterprise application attribute mapping

Question

I am trying to understand how to write an expression within my Enterprise Application attribute mapping section with the following requirement

I need to pass the following to the destination app (Salesforce Perms)

I want to take the givenName (first name) source attribute and the sn (last name) and pass the first letter of the givenName and the first 5 digits of the sn so:

Bob Smith should equal bsmith using the expression

Can someone give me any advice or an example that will give me the result.

Thanks in advance

looking here for reference so far

https://learn.microsoft.com/en-us/entra/identity/app-provisioning/functions-for-customizing-application-data