WASHINGTON — It did not take long for President Trump to see an opening during his July 25 call with Volodymyr Zelensky, the young new president of Ukraine.

Just after 9 a.m. in Washington, Mr. Zelensky was heaping praise on the American president for bragging about helping Ukraine in its yearslong war with Russian-backed separatists. “You are absolutely right. Not 100 percent, but actually 1,000 percent,” Mr. Zelensky gushed, according to a reconstructed transcript of the call the White House released on Wednesday.

When Mr. Zelensky said Ukraine was almost ready to purchase American Javelin anti-tank missiles so it could better repel armored assaults by Russian-supported fighters, Mr. Trump pounced.

“I would like you to do us a favor though,” Mr. Trump responded, beginning a series of pointed requests. The president pressed Mr. Zelensky to use the help of Attorney General William P. Barr in opening an investigation of a company involved in the beginnings of the F.B.I. inquiry of Russia’s 2016 election interference. He also wanted a corruption investigation connected to former Vice President Joseph R. Biden Jr., a Democratic rival.