How to set up a contract with a fixed component + success fee % on fundraising

[#1]

Hello, I work in the fundraising industry and would like to know how to best structure a contract that encompasses:

- a fixed retainer fee (monthly) or a Milestone based fee (this one, I assume it should be easy using Fixed price contracts)

- a success fee as a % of the funds I help the company raise (no timeline or amount is predictable at the moment, nor if they will be able to raise any money)

Thanks

[#2]

No default option for that. Use fixed-price with milestones + "add bonus" for additional.

[#3]

Alessandro,

I have my clients setup Milestones and when the Milestone is completed they can pay and Bonus.

[#4]

Thanks to both!