I've seen several solutions for passing variables via a URL string. I'm just wondering how you can find these variables. For example, there are some that have explained as below. I'm trying to find the 'Call Type' variable on a task to pass in the URL string. This one listed below is for just 'Type'.
Type: &tsk10={!text} (The text you type in here must be a value in the picklist)
example string: https://na6.salesforce.com/00T/e?title=Call&who_id=!User.Id}&00N800000004wDKu=Marketing&what_id={!Account.Id}&tsk3=!Account.Name}&tsk5=Call&tsk12=Completed&retURL=%2F{Account.Id}