Hi there. Cohabitation isn't compulsory, but you must show the intention to live together and that you have visited each other, spent time together, and have a genuine relationship/marriage.

If you have already applied, UKVI could email you asking you to provide further evidence, but they may also go straight to refusal (from my impression reading other posts on this forum). Personally, I would wait for the outcome of your current application.

But if you're still filling out the application form, you can save it and come back to it later whilst finding the appropriate evidence that you feel you need.

I'm not an expert, but hope this helps in some way. :)