Rest API help

Hello, I would like to use Rest API for creating requesters. When I execute code below in Powershell, I get message "No XML data specified". What's incorrect here?

$inputdata = @"
<?xml version="1.0" encoding="UTF-8"?>
<API version="1.0" locale="en">
    <records>
    <record>
        <parameter><name>Name</name><value>testfromapi</value></parameter>
        <parameter><name>CI Type</name><value>Requester</value></parameter>
        <parameter><name>Site</name><value>Common site</value></parameter>
        <parameter><name>Business Impact</name><value>Medium</value></parameter>
        <parameter><name>Description</name><value>New user joined</value></parameter>
        <parameter><name>Employee ID</name><value>1111</value></parameter>
        <parameter><name>E-mail</name><value>test@my.address</value></parameter>
        <parameter><name>Phone</name><value>11111</value></parameter>
        <parameter><name>Mobile</name><value>11111111</value></parameter>
        <parameter><name>Department Name</name><value>Finance</value></parameter>
        <parameter><name>Department's site</name><value>Test</value></parameter>
        <parameter><name>Job title</name><value>Testtitle</value></parameter>
    </record>
    </records>
</API>
"@

$URI = "http://servicedesk.domain.local:8080/api/cmdb/ci"
$postParams = @{OPERATION_NAME='Add';TECHNICIAN_KEY='my_api_key';INPUT_DATA=$inputdata}
Invoke-WebRequest -Uri $URI -Method POST -Body $postParams