Using Field name as select parameter in SQLDataSource

If what you mean is using values from a control [like textbox, dropdown] then here is something

<SelectParameters>
    <asp:ControlParameter ControlID="FieldKey" PropertyName="Text"
         Type="String" />
    <asp:ControlParameter ControlID="FieldName" PropertyName="Text"
         Type="String" />
</SelectParameters>

Assuming the values are stored in textboxes

Here is my hack(workaround) most time

protected void page_load(...)
{
   SqlDataSource1.SelectParameters["FieldKey"] = field;
   SqlDataSource1.SelectParameters["FieldName"] = name;
}

OR create two hidden fields, assign the values to those fields and then use

<asp:ControlParameter />

That is another hack

OPTION 1

If FieldKey and FieldName are taking from code behind I think you can easily set command as below.

SqlDataSource1.SelectCommand = string.Format("select * from sometable where {0} = {1}",fieldKey ,fieldValue)

please validate input fields for SQL injection attacks.

OPTION 2

Create stored procedure to accept two parameters key and value, you need to build SQL query from those parameters and execute inside stored procedure.

Then you can call it from datasource.

<asp:SqlDataSource ID="SqlDataSource1" runat="server" 
    ConnectionString="<%$ ConnectionStrings:DbConnection %>" 
    ProviderName="<%$ ConnectionStrings:DbConnection.ProviderName %>"      
    SelectCommand="StoredProcedureName"
    SelectCommandType="StoredProcedure">
<SelectParameters>
    <asp:Parameter Name="FieldKey" Type="String" />
    <asp:Parameter Name="FieldValue" Type="String" />
</SelectParameters>
</asp:SqlDataSource>