Replacing several cells + creating a keyboard shortcut

I've got a very very small request - I was hoping someone could donate 1 min of their time!

i want this:

if cell in column a = "abc def" then replace column j in the same row with "=otherWorksheet!a4"

i need to do selective replacements - i will otherwise have to do the above manually - hundreds of times

i need instructions on how to write the above code + how to make it into a macro that i could execute with a keyboard shortcut (or buttonpress)

apologies if the above request is a cheeky one - i know i could do myself - but that might involve a few hours of putting my head down

thanks in advance if anyone can help


om