It did not ‘have a string attached’.
He phrased a question as “Do us a favor and...” which they used to impeach him.
It was not “Investigate Joe Biden or ELSE”
Of course he did not say “Investigate Joe Biden or ELSE” In international affairs, threats need to be more subtle.
If I remember my history at the time of the phone call our President was delaying or withholding hundreds of millions of Ukraine defense aid dollars which had already been approved by Congress. You think Zelensky did not recognize a threat even if it was expressed as asking for a favor?