assuming you would like to search for a task name?
In your search microflow you can pass a string value with the search request. Then you can use a retrieve tasks activity with a contains function on task name functionality like
contains($Task/Name, 'searchString'). Then you have to return a list of matching tasks (linked to the search object, since you cannot pass a list as an output parameter in your microflow).
A second option will be to write custom widget like the search widget in the Mendix AppStore.