How to Write a Apex trigger on Event to autopopulate Title of Contacts in Salesforce?

Custom button for creating Event can pass url query parameters and prepopulate field values.

Something like this works for Task:
/00T/e?
followup=1&
retURL=%2F{!Contact.Id}&
who_id={!Contact.Id}&
what_id={!Account.Id}&
tsk5=LVM&tsk6=Left+a+voicemail+for+{!Contact.Name}
&tsk5_fu=FU+VM+w+{!Contact.Name}+fr+{!Account.Name}&
tsk4_fu={!TODAY()+2}&
00NG0000008GDEN=Phones&
00NG0000008GDEN_fu=Phones+FU&
save=x

and will replicate standard `Log A Call` button functionality to be under click of custom button `LVM`