I have a function which returns a URL from hyperlink as a first parameter to IMPORTXML function.
For the function to work I need to put the output of that function in quotes, how do I do it?
As pnuts said, you don't need quote marks there. Think of them as a container we have to use to separate our text from formulas, as in
= "not" & B3, so that the string is recognized as such. If the text comes from a formula, it is already known to be text.
That said, if you actually had to put quotes around formula output, that would be done with
"""" & linkURL(A2) & """"
The rule is that quote marks
" inside a string must be doubled. So,
"""" is a string with a single character
="""" & A2 & """" outputs
"url" if the cell A2 contains url.