ASP.NET Core Razor page - refresh calculated field?

I come from the old ASP.NET WebForms and I have a question on the Razor Pages. Since ASP.NET Web Core Razor Pages are rendered on the server how would you do a simple calculated field that is automatically updated when user changes values on the form on Razor Pages?

For Example a simple calculator like Value1 * Value2 = Total and the user changes the Value1 or Value2 and Total is instantly recalculated on the screen.  In the old WebForms I used to use a trick but how would you do this in Razor Pages?

Thank you