How To Get Name From Table And Insert It As ID To Another

I have an access database that contain 2 tables and 1 form :

table1 named customers (custid-custname)
table2 named sales(salesid-custid)
form named form1 contain (salesid-custid)
*note:in form1 custid is combobox

What i need is when i select an customer from combobox the column custid in sales table get custid not custname

I use this sql statement to get combobox items from customers table

Code:
SELECT Cust.[Cust-Name] FROM Cust;