Plus sign in create dashboard REST api

[hershs]

I trying to create dashboard via REST api.

The query string of the panel contains '+' character, like this

<dashboard>
    <row>
        <panel>
            <title>Alert Rate (percent of transactions)</title>
            <viz type="Splunk_ML_Toolkit.OutliersViz">
                <search>
                    <query>index=my_index ..... | eval upperBound=(avg + stdev*exact(1.5))</query>
...

Creating  the dashboard by calling

curl -k -u https://splunk_domain/servicesNS/admin/search/data/ui/views --data @./test.xml

After calling the dashboard created with expression 

index=my_index ..... | eval upperBound=(avg  stdev*exact(1.5))

The '+' disappeared

Please help!