Create constraint in SSMS but instead of Save click on Generate Script button

>I have a table where I added a default in SSMS. So the default has a constraint name (e.g. 'DF_table_name_col_name')
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>Now I wanted to drop this default constraint and create a new one; in code. Dropping it seems to be easy. But when I try to create the new default constraint in code I get error. Here is syntax I use:
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>alter table table_name alter column col_name char(20) not null add constraint [DF_table_name_col_name] default ('')
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>What is wrong with my syntax above? TIA.