You can append url parameters to set a wrapper based on the security category and wrapper id.

www.mysite.com/site/TellAFriend?pw_id=wrapper_id&s_affiliateSecCatId=catIDnumber

You can determine these id's by going to Setup > Page Wrapper Editor > Searching for the particular wrapper and determining the id, by clicking manage and viewing the pw_id number in the url.

Similar steps will result in determining the security category id number. Setup > Security Categories > Searching for the particular security category and click the name. In the url category= will provide the number.