Add custom element for link to twitter with handle

I’m using the new custom element to do the following and need help on configuring:

In my CMS, I have a field for the persons twitter handle.
Since all twitter URL’s are the same format (, I just want to dynamically insert the handle after the twitter address.

I was able to do this from a regular embed and dynamically insert the twitter handle. but I had to write a tiny bit of html.

I was hoping to use the Custom element to achieve this.
Once however, I add the attribute and select the field, it overrides anything I wrote in the attribute field.

Any ideas?