Requirement

I’d like to create a button on Opportunities that does several things:
1.) Closes the Opportunity (Stage = Closed Lost)
2.) Makes a custom field have a specific value (Opportunity Lost Reason = “Wrong/Bad Number”

Solution:
Here’s the URL behind the button:

/{!Opportunity.Id}/e?retURL={!Opportunity.Id}&opp11=”Closed Lost”&00N80000002eg5T=”Wrong/Bad Number”&save=x

Where

00N80000002eg5T is the ID of the “Opportuntiey Lost Reason” custom field!
opp11 is the SF field name for Stage standard field in the layout.
save=x will automatically save the record after the changes are made.