You can grant a user full control access to the user policy of a given web application using PowerShell with this script.  This script uses the following variables:

You can also grant different role types such as "FullRead", "DenyAll", or "DenyWrite" permissions as defined in SPPolicyRoleType.  For example use "FullRead" to grant you search crawl account readonly access.

PowerShell
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$webApplicationUrl = "http://server" 
$account = "domain\user" 
$roleName = "FullControl" 
 
$webApplication = Get-SPWebApplication -Identity $webApplicationUrl 
$account = (New-SPClaimsPrincipal -identity $account -identitytype 1).ToEncodedString() 
 
$zonePolicies = $webApplication.ZonePolicies("Default"$policy = $zonePolicies.Add($account$account$role = $webApplication.PolicyRoles.GetSpecialRole($roleName$policy.PolicyRoleBindings.Add($role$webApplication.Update()