If you need to create a new host with OP5 api you can use something like this.
Template, hostgroups and contactgroups are not mandatory and can be removed from hash table.

function new-op5host
(
	[parameter(Mandatory = $true)]
	[string]$hostname,
	[parameter(Mandatory = $true)]
	[string]$alias
)
{
	$username = 'api_user$Default'
	$password = "password"
	
	$base64AuthInfo = [Convert]::ToBase64String([Text.Encoding]::ASCII.GetBytes(("{0}:{1}" -f $username, $password)))
	
	$hash = @{
		file_id = "etc/hosts.cfg";
		host_name = "$hostname";
		address = "$hostname.domain.com";
		alias = "$alias";
		max_check_attempts = "3";
		notification_interval = "5";
		notification_options = "d", "r";
		notification_period = "24x7";
		template = "Win_hosts";
		hostgroups = "Windows servers";
		contact_groups = "Windowsservrar"
	}
	$jsonData = $hash | convertto-json
	
	$result = Invoke-RestMethod -Method POST -ContentType "application/json" -Uri "https://op5.domain.com/api/config/host" -Body $jsonData -Headers @{ Authorization = ("Basic {0}" -f $base64AuthInfo) }
	
	#Save
	$result = Invoke-RestMethod -Method POST -ContentType "application/json" -Uri "https://op5.domain.com/api/config/change" -Headers @{ Authorization = ("Basic {0}" -f $base64AuthInfo) }
		
}