Pandas excel insert data/url in A1 cell

I want to have an url in single cell (A1). My DataFrame is from python dict and it's without this url. But I have it under variable url. I've tried df.iat and it did not work.

df = pandas.DataFrame(data=collection)
df = df.sort_index(ascending=False)
df.to_excel(writer, sheet_name=sheetname)
df.iat[0, 0] = url

On picture, I've markup where the url should be.

Any ideas ?