You can start with this example:
This MWE sets the page geometry and removes \parindent
to get enough space for a A4 page to be included from the external pdf file.
Also \pagestyle is set to empty to prevent typesetting of the page number.
Command \includepdfmerge from the pdfpages package extracts the first page
from hello.pdf (any A4 pdf file, not included in the MWE)
and puts if into a vertical box by \vbox command.
\unitlength defines the coordinate units (mm) to be used by the picture environment
(default is 1pt). \vfill pushes the following picture to the bottom of the page,
to be able to use the bottom left corner of the page as a reference point (0,0).
The picture environment reserves space (200x200) for the picture drawing area
and applies the command from the question to put a text box on a green background
at the location (100,100) on top of the included pdf page.