As far as I'm aware, it's not currently possible from Inkscape.
$ inkscape --export-type=ico input.svg
InkFileExportCmd::do_export: Unknown export type: ico. Allowed values: [.dxf, .emf, .eps, .fxg, .gpl, .hpgl, .html, .jpg, .odg, .pdf, .png, .pov, .ps, .sif, .svg, .svgz, .tar, .tex, .tiff, .webp, .wmf, .xaml, .xcf, .zip]
If you just need to get an .ico file, maybe try exporting from GIMP.