[dba-SQLServer]Prompting for Value

Need hints, please.  I need to ask the user for a value for filtering
records in an sql statement.  Just a simple, what promo_id that I have
underlined in the select statement below.  Looking everywhere, must not be
asking the correct question.  I assume a variable is declared and then the
variable pops in the place of the underline?  Thanks in advance.  Happy
Monday.

SELECT prospect.prospect_no, prospect.prospect_name, 
    prospect.prospect_addr1, prospect.prospect_addr2, 
    prospect.city_name, prospect.state_id, prospect.zip_code, 
    prospect.branch_no, prospect.mktsrc_id, prospect.phone1, 
    prospect.prospstat_id, prospect.sales_emp_no, prospect.udf9, 
    prospect.udf10, prospect.promo_id, 
    prospect.referring_cust_no, prospect.udf7, 
    prospect_archive.prospstat_id AS prospstat_id_archive, 
    customer.cust_name, customer.cust_addr1, 
    customer.cust_addr2, customer.city_name AS Expr1, 
    customer.state_id AS Expr2, customer.zip_code AS Expr3, 
    customer.phone1 AS Expr4
FROM dbo.prospect INNER JOIN
    dbo.prospect_archive ON 
    dbo.prospect.prospect_no = dbo.prospect_archive.prospect_no LEFT
     OUTER JOIN
    dbo.customer ON 
    dbo.prospect.referring_cust_no = dbo.customer.cust_no
WHERE (dbo.prospect.promo_id = ____________________ )