Welcome to the forum!

1) Use layers.

Enter the up-stem notes (= the half) in Layer 1.

the down-stem notes (= the eighths) in Layer 2.

2) For the very first eighth you must change the notehead to the open notehead character used for halves.

To do so, use the Special Tools Tool, sub-tool Note Shape Tool.

Switch to Layer 2, and click in the measure/staff, to make handles appear on the noteheads.

Drag-select the handle on the first eighth, and hit the Return key, to open the Symbol Selection.

I do not know, neither what Finale version you are using, nor the music font you are using for noteheads, but the open notehead used for halves is (in most music fonts) in character slot #250.

I hope that this is clear?

If not, ask again.

I got it! Thank you very much! 

Merry Christmas.