Omission of する in ようとする when meaning "try to"


The aunt is trying to pull a jumper over Harry's head:

> ハリーの頭からかぶせようと、おばさんがやっきになればなるほど服はどんどん小さくなった。  
>  The harder she tried to pull it over his head the smaller it seemed to
> become (original text)

I'm struggling to parse this sentence.

I think this part is fine おばさんがやっきになればなるほど服はどんどん小さくなった = "the more desperate
the aunt became the smaller the clothes got." But I can't figure out how
ハリーの頭からかぶせようと fits into the sentence.

I would have been happy with ハリーの頭からかぶせようと **して**... or ハリーの頭からかぶせようと
**したが**... or something like that to means "she **tried** to pull the jumper
over Harry's head and/but ...". Is it simply that the する part is omitted (I
didn't know you could do that with "try to") or does this part connect to the
rest of the sentence in some other way?

