If plugin will be used for adding FBA user to site collection admins add the followg sections to app.config
(values here are only for example. Copy real values from FBA site's web.config):

<connectionStrings>
    <add name="TaloyhtioFBA_String" connectionString="Data Source=.;Initial Catalog=AspNetDb_Taloyhtio;Integrated Security=True" />
</connectionStrings>
<system.web>
    <membership defaultProvider="i">
        <providers>
        <add name="i" type="Microsoft.SharePoint.Administration.Claims.SPClaimsAuthMembershipProvider, Microsoft.SharePoint, Version=15.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" />
        <add connectionStringName="TaloyhtioFBA_String" name="Taloyhtio" enablePasswordRetrieval="false" enablePasswordReset="true" requiresQuestionAndAnswer="false" applicationName="/" requiresUniqueEmail="false" passwordFormat="Hashed" maxInvalidPasswordAttempts="100" minRequiredPasswordLength="1" minRequiredNonalphanumericCharacters="0" passwordAttemptWindow="10" passwordStrengthRegularExpression="" type="CustomMembershipProvider.CustomMembershipProvider, Taloyhtio.CustomMembershipProvider, Version=1.0.0.0, Culture=neutral, PublicKeyToken=ecbf947198042521" />
        </providers>
    </membership>
    <roleManager defaultProvider="c" enabled="true" cacheRolesInCookie="false">
        <providers>
        <add name="c" type="Microsoft.SharePoint.Administration.Claims.SPClaimsAuthRoleProvider, Microsoft.SharePoint, Version=15.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" />
        <add connectionStringName="TaloyhtioFBA_String" applicationName="/" name="Taloyhtio_Role" type="Taloyhtio.CustomRoleProvider.CustomRoleProvider, Taloyhtio.CustomRoleProvider, Version=1.0.0.0, Culture=neutral, PublicKeyToken=ecbf947198042521" />
        </providers>
    </roleManager>
</system.web>