You can pass search variables through a url link. To search a specific field use the name of the HTML input tag within the source of the search form as the name of variable passed and its value as the term you want to search that term for.
To do a basic search for your whole site
http://example.com/index.php?a=19&b[search_text]=searching+for+something&b[search_titles]=1&b[search_descriptions]=1
where "searching for something" is what you are searching for. Any values passed by URL must be urlencoded. Unless there are special characters the only thing you should need to do is replace " " (spaces) with "+" (plus signs)1).
To search a specific category (and its subcategories):
http://example.com/index.php?a=19&b[search_text]=searching+for+something&c=###&b[subcategories_also]=1
where ### is the category id number for the category you wish to search.
For basic text searches of fields…
http://www.yoursite.com/index.php?a=19&b[optional_field_1]=ford&b[optional_field_2]=green
For range searches on number only site wide fields…
http://example.com/index.php?a=19&b[optional_field_1_lower]=1970&b[optional_field_1_higher]=1975
The secret to creating these searches is duplicating the form in the variables it submits and the names used for those variables.
<tip c n>Note that URL's cannot have any spaces (ie " ") within them. If there is a space any data after the space within the url will be ignored.</tip>