It depends on what the source is of the string passed
In any case, the string value should be captured in a variable before the retrieve action.
This variable can be used to express the string value of the contains function.
This variable can be of type Object. In case a user expresses a value in a input field on a page. Hence you do have a dataview with an object and attributes you pass into this microflow.
Then the expression would be $NameOfTheInputVariable/AttributeUsed
Or this can be an object as a result of a previous microflow action (retrieve, create, rest service call, ....) Then the expression would be $NameOfTheActionResultVariable/AttributeUsed
Or this can be a string variable as a result of a previous action
Then the expression would be $NameOfTheStringVariable