If your mod uses or changes a file from another mod it is automatically a requisite. If your mod does not change or directly use files from another mod, you cannot force the mod to be a requisite. But you can always mark the mod as a required item on the Steam workshop page. But if it is not an actual requisite the mod loader will never think it as such.
You can make another mod as a required item, by opening both mods in the FCS and using one item from the other mod somewhere in your mod.

I think I understand. I was just trying to LOAD it as a req but it never auto selected in the fcs dialog because I haven't actually changed anything yet. Thanks!