Have a look at the demo for an example implementation.

You will need to implement the following things for this to work

1. Add local variables.
   1. Search term (Text)
   2. CurrentItemId (EntityId)
      
2. Use these in your query. Ensure you always include the current selected item, example below in red to always include the selected item
   1. Eg. 

      Sample_Request.RequestName like "%" + SearchTerm + "%" or Sample_Request.Id = CurrentRequestId
      

      
      

3. Implement ItemSelected to set the CurrentItemId, convert to the correct entity ID type
   

4. Implement SearchUpdated method to update local variable and refresh the query
   

5. Implement ClearItemClick, to set the current item back to null

   
   
   

6. Map the query to the input of the AsyncTypeahead WebBlock. Map the Id to the ID, and whatever you want as the displayName to the display name
   
   

7. That's it!
   If you have any issue send me through a question