Mobilize has an out-of-the-box donate-to-RSVP feature that is available to users with access to EveryAction Contribution Forms. If you do not have access, here is a workaround.
Create an Anytime Action form that links to an external site -- in this case, your donation page! Find a step-by-step guide in the Anytime Action article.
Create a Private virtual or in-person event following this guide.
In Edit view of the Anytime Action form corresponding to the private event, paste the link of the private event into the form's Private Details field.
How this works:
The public Anytime Action form can be distributed to collect donations.
Once a supporter clicks on the Anytime Action button, they'll receive the link to your private event that they can then register for.
You can match actual contributions to your registration list to ensure every attendee has donated.