CREATE TABLE [dbo].[Prescriptions] (
    [Prescription_ID]  INT           IDENTITY (1, 1) NOT NULL,
    [Doctor_UserName]  NVARCHAR (50) NOT NULL,
    [Patient_UserName] NVARCHAR (50) NOT NULL,
    [Drug_ID]          INT           NOT NULL,
    PRIMARY KEY CLUSTERED ([Prescription_ID] ASC)
);

CREATE PROCEDURE [dbo].[AddPrescription]
    @doctor nvarchar(50),
    @patient nvarchar(50),
    @drug nvarchar(MAX)
AS
insert into dbo.Prescriptions (Doctor_UserName,Patient_UserName,Drug_ID)
Select @doctor, @patient, d.Drug_ID
from dbo.Drug as d
where d.Trade_Name = @drug

<asp:SqlDataSource ID="SqlDataSource3" runat="server" ConnectionString="<%$ ConnectionStrings:DefaultConnection %>" SelectCommand="AddPrescription" SelectCommandType="StoredProcedure">
    <SelectParameters>
        <asp:Parameter Name="doctor" Type="String" />
        <asp:ControlParameter ControlID="PatientDropDown" Name="patient" PropertyName="SelectedValue" Type="String" />
        <asp:ControlParameter ControlID="PrescriptionDropDown" Name="drug" PropertyName="SelectedValue" Type="String" />
    </SelectParameters>
</asp:SqlDataSource>

protected void CreatePrescriptionBtn_Click(object sender, EventArgs e)
{
    SqlDataSource3.SelectParameters["doctor"].DefaultValue = System.Web.HttpContext.Current.User.Identity.Name.ToString();
    SqlDataSource3.DataBind();
}