Note, if you have to fill forms on your application, it is likely a "zeroth step" in the employment process. Its most probable function to silently dismiss the applications where it is some crap. The HR will probably ignore it, except that it will be a serious disadvantage, if you had written something which is a red flag for them.
In theory, you can write anything or you can leave it empty.
However, I would say to fill it. Without seeing the whole form, I suspect it has essentially a similar role like a motivation letter.
It is a way to show for them, that you consider them as an important option about your future.
Write some general sentences about that why would you like to work for them. Avoid anything looking "red flag". For example: "I found your job offer because I had a very good experience about X (as you can see in my previous work Y in my CV), and I would be very happy to work again in this environment", or similar.
Take care to have perfect grammar. If you are not a native speaker, find native speaker sentences with Google, and tune them. If you are a native speaker, still commit every possible to make it looking the best.
Note: your chances are better, if you invest double effort into 5 job offers, as if you invest single effort into 10!
P.s. filling such forms makes likely an "over-weighted HR syndrom" at the target company. These companies like to test an irrationally high count of candidates for their open positions. This makes your efforts in-effective, because their recruiting process doesn't target to find good employee effectively; instead it serves the goal of their HR team to show the need for their work.