I recently had a need to take a collection of data in a Nintex Workflow Cloud workflow and create an item in a SharePoint Online list, setting a choice field from my collection.
The structure that SPO needs it item1;#item2;#item3 etc. So I built that logic out. The workflow is below, but I also separate it out into a Component Workflow that you can reuse in your environment.
Nintex Workflow Cloud – Multiple Choice Field to SharePoint Workflow – ad43e6adde004834b9c64b27ac1dc089
Nintex Workflow Cloud – NWC Collection to SPO Choice Field Structure Component Workflow – a8b461fb863d4dc1834b53931026d3c3