You can pass any string/list of strings to your event handler by completing these two steps:

  1. Web.config definition:
    <handler type="Custom.TestEvent, PublishEvents" method="OnPublishBegin"> <mystring>teststring</mystring> </handler>

  2. In the code you should define a field named mystring and a property with get and set accessors:

private string mystring;

public string MyString
{ set { this.mystring = value; }
get { return this.mystring; }