What is the best way to create a drop-down list on a Dashboard where the user can select multiple values? And how can the multiple values be used as inputs in an OR statement in a DM query?
Regarding your first question it looks like the "options" is exactly what you need. This page Input Functions captures all the input types we have. Unfortunately for this one there is no real example, but the "options" is the drop-down allowing multiple selections (the "option" is the same with just one selection). Anyway it is a good overview about all the types you can use with links to the API doc where there is a description of the functions usually with examples etc.
Regarding your second question: Filter.in('Your_DM_Field', your_input_list)