If "Heading Section" is the name of a style, you will have to assign
it to TOC level 1; then each paragraph using this style will display
as a level 1 entry in the table of contents. The easiest way to do
this is to modify the field code of the TOC field. In other words, you
would need something like this:

f.Code.Text = "TOC \o \t ""Heading Section;1;my custom style;2"" \h"

(assuming of course that f is a valid reference to a Field object).

Note that if your list separator (Regional Optins in Control Panel) is
a comma, that's what you should be using in the field code above
(instead of a semi-colon).

If there is just one TOC field (corresponding to a single TOC), you
could easily locate the field in a loop:

For Each f In ActiveDocument.Fields
  If f.Type = 13 Then
     'Set the Text property as indicated above...
  End If
Next f

Then update the TOC:

ActiveDocument.TablesOfContents(1).Update