I want to implement auto suggest feature while typing the city name in textbox

If you search for the word "autocomplete" using the search box here on Tempo on the top left corner you will find several useful posts. One I found by doing that search is forum.appian.com/.../f-83518 where I even posted an example.

You can use the Advanced Form Utilities plugin, available under Shared Components for this. It has functions called attachAutocompleteFromExpression() and attachAutocompleteFromQuery which can be used for autocomplete. Please note that Javascript is not supported on mobile forms.