The correct form is "feedback".
The word "feedback" is a loanword from English and is used in Romanian without undergoing significant changes in its form. When it is need to transform it into a genitive-dative form (example: "of the feedback"), we add hyphen ("-") between "feedback" and "ul" to clearly indicate that "feedback" is a compound word made up of two parts.
Thus, the correct form is "feedback" when used in a context where reference is made to this specific concept of providing return information or ASSESSMENT.
Examples of correct usage:
Therefore, to clearly indicate the genitive-dative form, a hyphen ("-") is added before "ul" and we have "feedback-ul".