A custodian account - or as known in Swedish, Aktiedepå - is a share deposit.
Similar to banks, when you buy shares it needs to be delivered to a specific account that is regulated by a bank and under the supervision of the Securities Exchange Commission (Finansinspektion).
In the case where a company is doing a pre-IPO round through FundedByMe they tend to have plans to list on the stock market, allowing investors the opportunity to exit.
In order for them to list on the stock market they need to deliver the shares to their investors accounts.
In Sweden the biggest and most well-know trade platforms for buying and selling shares and funds are:
All these services are free to sign up for and an account can be created through their respective iOS or Android apps using Mobile Bank ID.
As part of the signup process you get an account with them know as a Share and Fund account (Aktie and Fond kontot). This account has a number associated with it and is usually required by the entrepreneur to successfully deliver your shares.
P.S. an ISK (Investeringsparkontot) AND KF (Kapitalförsäkring) is not a valid account in this case.