Good day all,
I was reading through the documentation for MP3Tag but couldn't quite grasp what I was doing.
But anyways, support is here 
Thank you in advance.
What I would like to do is create an action that would insert a character at the end of the %TITLE% tag field
As you do not tell us what character, see the FAQs example:
/t/967/1
which shows you how to insert 4 characters
No wonder I wonder I couldn't get it done .. I was using 'Replace with regular expresion" .. 
FAIL.
I got it now, muchos gracias!
In a way your question resembled this one (which you asked some time ago):
Add charachters to ARTIST field