Combo box is not showing record
The formula itself should work if the structure of Items matches {dk_emailsequencenumber: X}. If Items remains as a simple list [1,2,3,4,5,6], then the formula won't match, and you'll need to either adjust Items or modify DefaultSelectedItems to work with that structure.
Modify Items to use records, try changing Items to match the structure expected by DefaultSelectedItems. For example, set Items to something like:
[
{dk_emailsequencenumber: 1},
{dk_emailsequencenumber: 2},
{dk_emailsequencenumber: 3},
{dk_emailsequencenumber: 4},
{dk_emailsequencenumber: 5},
{dk_emailsequencenumber: 6}
]
Maybe this will help solve your issue.