Query string is a tail added onto a URL that carry information from one page to another page. To understand query strings, let’s first take a closer look at a sample URL that uses one:
https://example.com/product/shirt/?utm_source=newsletter&utm_medium=email&utm_campaign=winter
The first section of a URL is called a protocol (https). It is followed by the domain name of the website, or the hostname (example.com). And the next part (after the forward slash) is the path (product/shirt/?).
Everything that follows the path after the question mark (?) is the tail of the URL, also known as the query string (utm_source=newsletter&utm_medium=email&utm_campaign=winter).
MailBluster has the option to put a query string in your campaign email, which you will find under Edit campaign > Content section:

If you set query string in your campaign, it will be appended to all the links of the campaign.