It is sometimes necessary to supplement parts of an existing window to provide the complete surface required for attaching the tape and membrane.
Attach a fixed supplementary frame
Use a length of square or rectangular rigid material such as timber or aluminium (eg. 30x20mm or 15x15mm) to make a second frame within the window reveal. Attach the timber/aluminium to the window reveal using glue and small screws or nails. Attach the tape and membrane to the new frame as for a standard window.