For a Forms page, you can set a Default Value for a Salesforce Element. This value can either be text you define or a variable that references information specific to the visitor's current session, such as values associated with a logged in user, if your site has User Integration enabled, or a user-defined session variable.
To do so, add the text or variable as the Default Value for a Salesforce Element in a Forms page. Text should be entered exactly as you wish it to appear. Variables will always have their name surrounded by curly brackets and begin with a dollar sign such as {$Variablename}. Only text or a single variable can be used as the Default Value. Multiple variables or a mix of text and variable cannot be used:
For a list of the possible variables and the situations in which they can be used, refer to the following articles: