You can get the value using this fetch:
<fetch version="1.0" output-format="xml-platform" mapping="logical" distinct="true">
<entity name="businessunit">
<attribute name="name" />
<attribute name="address1_telephone1" />
<attribute name="websiteurl" />
<attribute name="parentbusinessunitid" />
<attribute name="businessunitid" />
<order attribute="name" descending="false" />
<link-entity name="systemuser" from="businessunitid" to="businessunitid" link-type="inner" alias="ac">
<filter type="and">
<condition attribute="internalemailaddress" operator="eq" value="Put your email id here" />
</filter>
</link-entity>
</entity>
</fetch>
