Assuming you are using the iControl PowerShell cmdlet's, here's something that should get you going.
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function Associate-MonitorWithPool()
Description:
This function will associate a monitor template with the specified pool.
Parameters:
PoolName - The Pool name to target.
MonitorName - The Monitor Template name to associate with the given pool.
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{
param(
[string]$PoolName,
[string]$MonitorName
);
$monitor_association = New-Object -TypeName iControl.LocalLBPoolMonitorAssociation;
$monitor_association.pool_name = $PoolName;
$monitor_association.monitor_rule = New-Object -TypeName iControl.LocalLBMonitorRule;
$monitor_association.monitor_rule.type = "MONITOR_RULE_TYPE_SINGLE";
$monitor_association.monitor_rule.quorum = 0;
$monitor_association.monitor_rule.monitor_templates = (, $MonitorName);
$(Get-F5.iControl).LocalLBPool.set_monitor_association(
(, $monitor_association)
);
Write-Host "Monitor '$MonitorName' Is Associated With Pool '$PoolName'";
}