(isearch-forward nil t)
(isearch-yank-string "PATTERN")

$ emacs FILE --eval '(isearch-forward nil t)'  --eval '(isearch-yank-string "PATTERN")'

(defun my-search-for (pattern)
  "Search for PATTERN with Isearch."
  (interactive "sSearch for: ")
  (isearch-forward nil t)
  (isearch-yank-string pattern))

$ emacs FILE --eval '(my-search-for "also")'