Hi,
This links has complet details as required: http://www.dotnetmafia.com/blogs/dotnettipoftheday/archive/2010/05/13/wildcard-search-web-part-for-sharepoint-2010.aspx
<Taken from the above link to refer:::>
- We start by adding assembly references to Microsoft.Office.Server.Search.
- Then we create a new web part inheriting from CoreResultsWebPart and add the following using statements.
using Microsoft.Office.Server.Search.Query;
using Microsoft.Office.Server.Search.WebControls;
-We then override the GetXPathNavigator method and get a reference to the QueryManager and override the UserQuery property. In reality, there are only two lines of code involved.
QueryManager queryManager = SharedQueryManager.GetInstance(this.Page).QueryManager;
queryManager.UserQuery = string.Format("{0}", queryManager.UserQuery);
Note: As per your requirement, try the above line as given below (Replace the managedpropertyname with the your custom one. I havent tried, but seems it should work.)
queryManager.UserQuery = string.Format("ManagedPropertyName:{0}", queryManager.UserQuery);
Share your results.