Easiest way is to run GPEDIT.msc and change the search list value under:

Computer Configuration\Administrative Templates\Network\DNS Client

Should be formatted like:  domain.com,nextdomain.com,lastdomain.com

Then run a GPUPDATE /force command

Registry Keys where the searchlist may be stored

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters

HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows NT\DNSClient\"SearchList"="domain1,domain2,domain3"

HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Policies\Microsoft\Windows NT\DNSClient\"SearchList"="domain1,domain2,domain3"