To Use the Newly Created Named Expression to Name Markups

Open a DGN, i-model, or overlay file.
Open the Markups dialog.
Click the Markup Settings icon to open the Markup Settings dialog.
Expand the Markup Name option button.
The first entry is Default. After a separator, Markup_<model> is listed.
Select Markup_<model>.
The field below the option menu shows Markup_Model01.
(Optional) Create more markups, and notice the generated markup names.
Markups are named Markup_Model01, Markup_Model02, and so on.

Note: While creating markups, notice the Create Markup tool settings. It contains a Name option button that let you temporarily override (for just this markup) the name/expression from the Markup Settings dialog.