How to get relevant work experience without relevant work experience

Hi everyone! Is it possible to get a summer internship in Investment Banking without relevant work experience? 

I'm a third-year Economics undergraduate student in Spain with great grades (9.5/10, top of my class) but no relevant work experience. I have done some volunteering at a startup fair and I am currently working as a (paid) research assistant after being selected by one of my professors, but nothing related to finance. I've also learnt financial modeling and valuation on my own by reading books and doing case studies, but have no way to prove it to a recruiter. US and UK universities seem to encourage students to start acquiring some experience early, but this is not the case in Spain (at least definitely not at my university).

How likely is it for me to land a summer internship without finance-related experience? Aren't summer internships supposed to be that first finance work experience?

Thank you and sorry for my ignorance!