In check settings there is an option to block URL's. This field is commonly used to block URL's that aren't essential to the check. An example would be google analytics, ad tracking software, etc. These URL's could cause the check to fail so blocking them could eliminate false negatives, i.e. a failed check run that actually ran just fine. This post references URL blocking syntax in different browser versions:

FireFox 46 & Chrome 55:

Firefox 46 and Chrome 55 are browser options for private agents only, as all public agent locations were upgraded to utilize evergreen Chrome versions starting in 2020. They support regular expressions for URL blocking. The example above will block any variation of google. The .* allows any characters which means it will block www.google.com, www.123google.com, www.123google567.com, and so on. ASM will tell you whether the entered regular expression is invalid when you try to add the rule. 

Chrome 74:

Chrome 74 supports wildcards instead of regular expressions. The example above will also block any variation of google. The * allows any characters just like .* did in the previous example. So if you're upgrading your checks to Chrome 74 make sure to change your blocks to wildcard.

Helpful Regular Expression Syntax for FF46/Chrome 55: