How to add field merge to a query string

Hello,

I've been working on methods to set up multiple query string URLs that also include a field merge to prepopulate the value for that paramenter so that the value will consequently be populated on a form field.

I've reviewed this article and it's confirmed that it is possible to add field merge to the query string. However, I'm having difficulty finding instructions on properly writing the URL that includes the field merge.

For example, if the template is

https://go.landingpage.com/vanityURL?mykey=<field merge>

My question is: what exactly is the correct syntax to replace the "field merge" within that "<>" in that template above?