pass optional parameters to webservice

I want website X calling my webservice to only need to provide relevant parameters.
for example: my webservice only saves userdata that needs to be saved, website X calls my webservice with values to save: firstname, lastname and country, another website instructs my webservice to save only firstname and lastname.

What would be the best approach for defining my webservice?

My current signature looks like underneath, but this forces a website to always define A value (even its just "") for parameters that are not used. I want websites to be able to call my webservice with only those parameters they really need. How would I make that possible?

<WebMethod()> _
Public Function SaveUserDetails(ByVal firstname As String, ByVal lastname As String, ByVal country As String) as Boolean
End Function




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