Tuplet

This is a little too small a snippet to be sure what is meant. Is this in a 6/4 time signature? If so it would mean you have 12 notes in a dotted minim, i.e. you don't need the dodecaplet marking at all, since the rhythm would be just normal demiquavers.

Generally what you look at is covered under "tremolo" and explained well in the handbook. If you can combine tuplets with tremolo I wouldn't know. But since you probably don't need the tuplet marking that may not be a problem. Or you can just "manufacture" the marking from the symbols palette if you want it there anyway.

Try a custom 12 / 8 tuplet on a half rest (in a 4/4 measure).
1.) You then can have two dotted-quarter notes/chords in a 12-tuplet. With that, you can add the two-line 16th tremolo between them (my understanding of having those half-note heads with a 16th beam between them is for tremolo). Thing is, the tremolo beams won't be connected but will be disconnected between the notes, at least that's how it looks in Musescore.

2.) If you want the connected beams between the notes, then you can sacrifice tremolo playback for the visual to be the same by instead of a 12/8 tuplet have a 12/32 tuplet under the same circumstances. You can then have two dotted 16th notes in a 12-tuplet and then change the note-heads to be half-notes in the inspector. There may be another way I'm unaware of, but this is the best I could do at the moment.

Here's the result (first measure is from 1.), second is 2.)