How Can I Include a Variable in an ID Value?

I have some code I would like to repeat in my design. So I have made it an SSI and I can include it in two different places.

But…

There are some items like forms and buttons and inputs which need a unique ID.

Can I define their ID to be something like this?

id="'this_name'+some_variable.value"

Or is there another way of achieving what I want?