Off topic:Create a column with the same name

Question

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I like to create a column with the same name for all rows, but I do not know how can I.

input:

chr1    875450  875756  
chr1    935086  936289  
chr1    948744  949058

output:

chr1    875450  875756  a
chr1    935086  936289  a
chr1    948744  949058  a