The great thing about the F6 is that you can toggle the SC Source of bands individually to EXT. To do this, place the F6 on your melody track, and route the vocal to the F6 as an external sidechain. If there is some unwanted frequency overlap, use an EQ like the F6 Floating-Band Dynamic EQ to carve out space in the melody for your vocals.
When it comes to the sound design of the melody you select, choose a sound that won’t conflict with vocals in the 1-5 kHz range this is where the intelligibility of vocals comes from, so it’s best to avoid frequency masking if possible. It’s easy to over-produce a beat when you don’t have vocals filling space in your mix, so anticipate the addition of vocals and arrange your song accordingly. Keep in mind that you’ll be adding other elements to your beat, so the melody should be catchy, but it doesn’t need to carry the entire weight of the song.