Early age startup Marketing role.

Simple answer, Don't - skip it, no matter how good company is.

You are de-valuing your experience, further if they really want you then they will match the offer you asked for.

This is very tricky.
How about questions regarding timeline to get the salary at market level? What sort of assurances can they give regarding the same? Perhaps a paper trail/something in written for the worst case scenario. Quite the trend, riding on the back of a funding winter. They want work to get done but don't want to pay fairly or pay at all.

If you have that conviction that the risk will pay off, well and good. But still, be careful.
If the business model is solid, PMF is sorted, that's at least something to be hopeful about. But meandering across business pivots, constant churn are as bad as they come.