If you look carefully at the Arabic and the English phrases, the word (office: مكتب) is indefinite. In Arabic it does not have (AL), and in English it is preceded by (a), and attributive adjectives. Anyway, since it is about typing in English, checking that the Arabic sentence does not define (office) then we must have (a). Your sentence defines "office" by virtue of using "this" which would be translated as: هذا المكتب instead.
Could you please add for هٰذا to be typed as هذا or هاذا? As it's not recognized as correct.
Unfortunately, many people complained about it but still they didn't fix it. However, هاذا cannot deemed to be correct. But they should accept هذا as a valid answer. Let's hope they fix it soon
well, so far I've discovered few sentences with the same issue. As for هاذا to be correct or not is a debatable question in arabic. Technically, pronunciation wise it's correct. But for nowadays writing هذا is used. http://www.alfaseeh.com/vb/showthread.php?t=89295 https://www.ahlalhdeeth.com/vb/showthread.php?t=31469
Away from debates for those scholars (and a large number of them are religious scholars more than linguists), this is the norm in education. The same case with the debate about the nature of the nominal sentence and its naming and nature, still, the education system works on classifying the sentence with its beginning.
Here we have learners .. new comers to Arabic .... it's better to let them be in line with the "school" system instead of put them astray on a complicated path of scholastic studies that even an average Arab does not know much about. The norm is using هذا.