To pre-populate Source object fields you should pass the fields values to the form parameters using fields API names.

On the example below you can see how could be configured the Parameters in case if you adding a form to the Lightning page. The form is configured on standard Event object.

Another example demonstrate how you can configure the form input Parameters in case if you are using the form "Navigate" action type .

To pre-populate form Virtual fields you should use "Default Value" field option.

The format of the pre-populating value could be different based on the Virtual field type. Please look at the article, linked with the your Virtual field type, in the proper chapter.