"I hate going to the car boot sale."
Translation:Dw i'n casáu mynd i'r sêl cist car.
No a with an acute angle is provided in the set of special characters below the answer box - impossible to get "casau" right.
á will be available with some sort of combination of keys on a computer - on MacOS with a Welsh keyboard layout chosen it is 'alt-7, a' for example - or on a phone/tablet you might get access to it by holding down 'a' for long enough to get a pop-up menu of accented forms of 'a'. There will be instructions on the web somewhere.