thank you for the quick response. It still doesn't work.
First I head the searchbox in a usercontrol on the masterpage. And with previouspage.findcontrol it didn't work.
Then I used the @mastertype and still it doesn't work. Also with putting the property it doesn't work. Can you please help me out.
This is the error message I get:
Validation of viewstate MAC failed. If this application is hosted by a Web Farm or cluster, ensure that <machineKey> configuration specifies the same validationKey and validation algorithm. AutoGenerate cannot be used in a cluster.