org agenda keyword prompty

Question

I've got a file with a lot of headlines, each of which has the keyword property defined. The basic case is that an entry has multiple keywords.

I'd like to define a custom agenda command that asks for an input keyword and displays all and only the headlines that have that keyword as one of its keywords. I can't figure out how to define a custom agenda command that asks for an input and then performs operations on/with it.

I know how to hardwire that the command should search only one of my .org files, rather than all of the files that are listed in agenda list. I.e., I know to start thus:

(setq org-agenda-custom-commands
      (quote
       (;this parenthesis is the beginning of the list of agenda custom views
    ("k" "Keyword Search in the Zettelkasten"
     ( ; start list of commands

      ;; command 
      ( ;; presumably the commands asking for user input go here (??)
          (              ; begin list of options
           (org-agenda-files '("~/org/zettelkasten.org")) ; only use zettelkasten
           )                 ; end list of options
          )                  ; end of scheduling command

      ) ;end list of commands
     (  ; begin view-wide options
      (org-agenda-compact-blocks t) ; compact view (this is just there to
                    ; show the place for view-wide options.
      ) ; end view-wide options
     )  ; end of definition of of this view 
    ;; The following parentheses are for the whole defintion of the variable
         )      ; marks the end of the list of views
    )       ; end of the quote needed to correctly set the variable
       )        ; end of setq command