@Shay Hayo here's an example SOAP request with the UserId predicate:
xml
<s:Envelope xmlns:s="http://schemas.xmlsoap.org/soap/envelope/">
<s:Header>
<Action s:mustUnderstand="1" xmlns="http://schemas.microsoft.com/ws/2005/05/addressing/none">SearchAccounts</Action>
<h:ApplicationToken i:nil="true" xmlns:i="http://www.w3.org/2001/XMLSchema-instance" xmlns:h="https://bingads.microsoft.com/Customer/v13" />
<h:AuthenticationToken xmlns:h="https://bingads.microsoft.com/Customer/v13">* * *</h:AuthenticationToken>
<h:DeveloperToken xmlns:h="https://bingads.microsoft.com/Customer/v13">* * *</h:DeveloperToken>
<h:Password i:nil="true" xmlns:i="http://www.w3.org/2001/XMLSchema-instance" xmlns:h="https://bingads.microsoft.com/Customer/v13" />
<h:UserName i:nil="true" xmlns:i="http://www.w3.org/2001/XMLSchema-instance" xmlns:h="https://bingads.microsoft.com/Customer/v13" />
</s:Header>
<s:Body>
<SearchAccountsRequest xmlns="https://bingads.microsoft.com/Customer/v13">
<Predicates xmlns:d4p1="https://bingads.microsoft.com/Customer/v13/Entities" xmlns:i="http://www.w3.org/2001/XMLSchema-instance">
<d4p1:Predicate>
<d4p1:Field>UserId</d4p1:Field>
<d4p1:Operator>Equals</d4p1:Operator>
<d4p1:Value>* * *</d4p1:Value>
</d4p1:Predicate>
</Predicates>
<Ordering xmlns:d4p1="https://bingads.microsoft.com/Customer/v13/Entities" i:nil="true" xmlns:i="http://www.w3.org/2001/XMLSchema-instance" />
<PageInfo xmlns:d4p1="https://bingads.microsoft.com/Customer/v13/Entities" xmlns:i="http://www.w3.org/2001/XMLSchema-instance">
<d4p1:Index>0</d4p1:Index>
<d4p1:Size>100</d4p1:Size>
</PageInfo>
</SearchAccountsRequest>
</s:Body>
</s:Envelope>
To get the user ID, first call GetUser. For more details please see the get started guide i.e., step #5 here.