If you have a Forms page, you can create links to that Forms page which include pre-defined values set in that URL. This is sometimes called pre-fill. Any field included on the Forms page can be used whether it is hidden or not, provided it doesn't have a Default Value identified.
For the field you wish to insert a value from the url, use the Salesforce Name. This name can be viewed in the Forms page's Form tab in the Element column or by viewing the rendered HTML of the form and locating the id for the field.
For an example of the formatting for the full URL, review this link that includes the name Mickey as the First Name:
To include multiple variables in the url string, simply include an ampersand between them like this:
NOTE: Including values in this manner is particularly helpful when sending mass email correspondence through a tool like Soapbox Mailer for Salesforce with a call to action that leads the recipient to a Forms page.