API to update LastLoginUsername in Custom Field

In an effort to help someone out with this who may be struggling as well, below is what I finally got to work. If you combine all of these script blocks together and ensure that you change out specific values to match your environment (e.g., $un, $pw, $customfield), this should work for you as well.

Authentication:

$un = 'your api id'
$pw = 'your api key'

$token = [Convert]::ToBase64String([Text.Encoding]::ASCII.GetBytes(("{0}:{1}" -f $un,$pw)))

$header = @{
'Authorization' = 'Basic ' + $token
}

$uri = "https://secure.logmein.com/public-api/v1/authentication"

$auth = Invoke-WebRequest -method get -h $header -uri $uri

$newheader = @{
    'Authorization' = 'Basic ' + $token
    'Content-Type' = 'application/json'
}

Get all your computers into a variable:

$hosts_uri = "https://secure.logmein.com/public-api/v2/hosts"

$hosts = invoke-webrequest -method get -h $newheader -uri $hosts_uri

$hosts_out = (convertfrom-json $hosts.content).hosts

$host_ids = $hosts_out.id

Generate your "system inventory token" thing:

$systoken_uri = "https://secure.logmein.com/public-api/v1/inventory/system/reports"

$newbody = @{	
    "hostIds" = @($host_ids)
    'fields' = @('lastLogonUserName')
    }

$systoken = invoke-webrequest -method post -h $newheader -uri $systoken_uri -body (convertto-json $newbody)

$systoken_out = (convertfrom-json $systoken.content).token

Then run your system inventory report. This only returns a subset of the results and I couldn't find a way to get it to stop, so I just set a timer (the .3 of a minute). You could increase this or let me know when you find a way to identify the script ending.

Along with adding some "replace" parameters to get rid of extra symbols in the data, I also had to add a "unique" sort filter at the end to clean up the duplicates entries that this method generates into the variable:

$sysinv_uri = "https://secure.logmein.com/public-api/v1/inventory/system/reports/$systoken_out"

$sysinv_out = @()

$timestart = Get-Date
$timeend = $timestart.addminutes(.3)

do {
    $sysinv_out_host = @()
    $sysinv_out_user = @() 

    $sysinv = invoke-webrequest -method get -h $newheader -uri $sysinv_uri

    $sysinv_out_host = ($sysinv.rawcontent -split "`n" -match "hostid" -replace '"hostID": ','' -replace ',','').trim()
    $sysinv_out_user = ($sysinv.rawcontent -split "`n" -match "lastLogonUserName" -replace '"lastLogonUserName": ','' -replace ',','' -replace """","" -replace "\\\\","\").trim()

    [int]$max = $sysinv_out_host.count
    if ([int]$sysinv_out_user.count -gt [int]$sysinv_out_host.count) { $max = $sysinv_out_user.count; }

    $sysinv_out += for ($i = 0; $i -lt $max; $i++)
    {
        write-verbose "$($sysinv_out_host[$i]),$($sysinv_out_user[$i])"
        [PSCustomObject]@{
            hostid = $sysinv_out_host[$i]
            lastuser = $sysinv_out_user[$i]
        }
    }

    $timenow = get-date

} until ($timenow -ge $timeend)

$sysinv_out = $sysinv_out | Sort-Object * -Unique

Finally, take that data you painstakingly combined into a variable and apply it to each of your machines, specifying the custom field ID that you want to write to:

$customfield = 'your custom field id'

foreach ($i in $sysinv_out) {

    $cf_update_uri = "https://secure.logmein.com/public-api/v1/hosts/$($i.hostid)/custom-fields/value-by-category/$customfield"

    $newbody = @{
        'value' = $i.lastuser
    }

    invoke-webrequest -method post -h $newheader -uri $cf_update_uri -body (convertto-json $newbody)

}

Make sure you go to your Computers tab after this finishes and turn on your custom field in your view. The search bar now not only searches computer names, but the last logged in user (since you last ran the script).