MicroStrategy pays interest on their debt via a mixture of their software business cashflow, existing reserves & short-term investments, credit and Bitcoin sales.

MSTR's annual coupon payments are not that high, maybe ≈$40M/year. The convertible bonds that they issue come with small coupons (e.g. 0%, 0.625%, %0.875, 2.25%)

Given MSTR's software business revenue of ≈500M/year - that portion is more that enough to cover the interest/coupon payments.