There are examples for Swift and C#, how to convert your Certificate to base64 encoded string.

1. Example function in Swift programming language:

import Foundation

func certificateToString(certificate: SecCertificate) - > String {
    return (SecCertificateCopyData(certificate) as Data).base64EncodedString()
}

2. Example function in C# programming language:

using System.Security.Cryptography.X509Certificates;

string certificateToString(X509Certificate2 certificate) {
    byte[] dataToEncode = certificate.Export(X509ContentType.Cert);
    return Convert.ToBase64String(dataToEncode);
}