I have the following author: “Alexander von Humboldt Foundation”. I’d like to use an acronym in the text instead of the full name to make the text more legible.
For instance, I’d like to have in my text: “AvH, 2009” rather than “Alexander von Humboldt Foundation, 2009”.
Assuming you want the full author name to appear in the reference list, I would suggest creating a new ref type for those kind of records. Put the acronym in the Author field and have another custom field for the full name. Use the custom field in the output style ref type template instead of the author field. This will mean creating a new ref type in the edit preferences options, and then creating a bibliography template for the ref type. I suggest you consult the help files or manual or training webinars, or posts by Crazygecko (who might have a pointer to a relevant thread) for the details.
You might also want to check how the referencing style you are using handles this situation, eg for APA with in-text citations
first citation = (Alexander von Humboldt Foundation [AvH], 2009)
later citations = (AvH, 2009).
If you are only citing the author a few times, it would be easier to leave the author in full in the EndNote reference and then edit the in-text citation in Word each time, excluding author and putting the required details in as a prefix.