First extract the MSI and MST File from the Redgate Installer with the Command
SQL_Search.exe extract C:YourFolder
Then you can perform a silent install
SQL Search_3.1.6.2050_x86.msi /qn /L* "%temp%Red Gate SQL Search 3.1.6.2050.txt" /norestart ALLUSERS=2 TRANSFORMS="SQL Search_3.1.6.2050_x86.mst"