How to Execute Sql UDF function

A scalar function is basically an expression. You use it in a query just as you would a column name, variable, or formula. Obviously you have to pass parameters to it in the query.

select colA, colB, @amt, colA+colB, dbo.ufNvarcharToAscii(colC)

Although this is outside the scope of your question, a scalar function should not be confused with a table-valued function. A scalar function produces a single result value, while a table-valued function can produce a set of rows and is used either like a table, or in conjunction with APPLY. Inline table-valued functions are treated much like views by the optimizer and can produce performance benefits over scalar functions when applied to a large number of rows.

select * from dbo.tfnListOpenOrdersForCustomerNo(custno)

select c.custNo,c.custname,t.orderno
from dbo.Customers c
cross apply dbo.tfnListOpenOrdersForCustomerNo(custno) as t

Say you have created that UDF - then you would call it with

SELECT schema.functionname(parameter)

So you might say

SELECT @asciiValue = dbo.ConvertToAscii(@nvarcharValue)

However, in this case, you would be well advised to use the ASC built in function.