I haven’t opened your link (I’m on mobile) but I think you want to set the Type of the RG to text, and the source to the value of a custom state list.
Workflow: when dropdown is changed > set state: choose element to hold state, create state, name it “selections”, type text, check list > value of state = element’s selections :plus item this dropdown’s value
The text in the RG would be “current cell’s text”
To clear, set the same state but leave the value blank.
So what you’re doing is creating a temporary list. Every time you make a dropdown selection, it adds that selection to the list.
If I’m misunderstanding and you’re working with a list field in your database, similar principles apply, but instead of set state, you’ll make a change to Thing and change the value of the list field: “[field] add dropdown 's value.” The RG here would be whatever type the Thing is and source would be a Thing’s list field. To clear, use “clear list” in the make a change action.
Gaby | Coaching Bubble