How to retrieve values from table based on input in textbox

One of the best ways to figure out how to do something is to look at another place in the system that does what you want or similar to what you want. Then use code coverage or debugger to follow how it was done.

This can easily be seen on the sales order.
When you enter the sell-to customer number it will fill the name, address, etc into fields based on that number.

sounds like what you are looking for.

These values are filled using code on the OnValidate trigger of the "sell-to customer" field in the sales header table.

that's where you will see how it was done.