I'm not sure what you mean by autocomplete dropdown validation? Autocomplete and Dropdown are different controls, and validation is achieved using Validate rule. A good place to learn how to use UI controls is to visit the UI gallery from the DesignerStudio landing pages. PDN also provides good resources at https://pdn.pega.com/user-interface.
I have a doubt regarding Dropdown vs AutoComplete. In general when we need to add a field to a form that provides a large list of dynamic values to chose from. What should be used? (Dropdown or AutoComplete)
Other Way of question - We need to select from a large list of dynamically populated values then we give the control as? (Dropdown or AutoComplete)
Please let me know the best option for above need.