How to specify rewrite url for query string parameters in Azure API Management

I'm using the Azure API Management to transform the incoming query string into another query string.

My transformation code is:

<policies>
    <inbound>
        <rewrite-uri template="api/primes?a={a}&b={b}" />
        <base />
    </inbound>
    <backend>
        <base />
    </backend>
    <outbound>
        <base />
    </outbound>
    <on-error>
        <base />
    </on-error>
</policies>

When I try to save the edits, the error appears:

One or more fields contain incorrect values: 
'=' is an unexpected token. The expected token is ';'. Line 15, position 50.

which refers to the equals symbol as in a={a}. How do I correct the template of the rewrite-uri? The input url is for example https://example.com/sum?a=7&b=5.

Try replacing:

<rewrite-uri template="api/primes?a={a}&b={b}" />

With:

<rewrite-uri template="api/primes?a={a}&amp;b={b}" />

Find more details at https://azure.microsoft.com/en-us/blog/policy-expressions-in-azure-api-management/.