How to create a text variable and then add a comma or period to that variable in an output text field?

I'm new to this, so please be patient with me. This is probably a stupid question, but here goes...

Storyline 3.

I want my user to enter text into a text field. That text is saved to a variable, firstName.

Now, I want that variable to display elsewhere, but I want that variable to display followed immediately by other text characters, depending on the context. 

So the variable firstName = "Jim"

On slide 1, I want to display that variable in a text field, such as Hell0, [text field]Jim.[/text field] (notice the period added) Or on Slide 3, Greetings Jim, (with the comma added). 

I thought that I could do this by specifying firstName followed immediately by a comma in the target text field, for example  %firstName%, but it's not working (the field is populated only by the contents of the variable "Jim" and the comma is gone).

I feel like I'm missing something super obvious... but I can't seem to figure out what it might be.

Can anyone help me?

Thanks,

Bill