When you take a closer look at the help:
Set-VMHostNetwork [-Network] <VMHostNetworkInfo[]> [-ConsoleGateway <String>] [-VMKernelGateway <String>] [-VMKernelGatewayDevice <String>] [-ConsoleGatewayDevice <String>] [-DomainName <String>] [-HostName <String>] [-DnsFromDhcp [<Boolean>]] [-DnsDhcpDevice <Object>] [-DnsAddress <String[]>][-SearchDomain
<String[]>] [-IPv6Enabled [<Boolean>]] [-ConsoleV6Gateway <String>] [-ConsoleV6GatewayDevice <String>] [-VMKernelV6Gateway <String>] [-VMKernelV6GatewayDevice <String>] [-WhatIf] [-Confirm] [<CommonParameters>]
It mentions <String[]>which means, it could be a single string or an array with multiple strings.
For example:
Get-VMHostNetwork -VMHost (Get-VMHost -Name "Your ESXi") | Set-VMHostNetwork -DnsAddress "10.10.1.1" -SearchDomain "test.com"
This will set DNS address to 10.10.1.1 with custom suffix to be test.com.
Get-VMHostNetwork -VMHost (Get-VMHost -Name "Your ESXi") | Set-VMHostNetwork -DnsAddress @("10.10.1.1","10.10.1.2") -SearchDomain @("test.com","test.test.com")
This will set DNS address to 10.10.1.1 and 10.10.1.2 with custom suffix to be both test.com and test.test.com
Not sure I understood correctly but hope this helps.
Steven.