This is what I do, I have a list control which lists the members in the front end if the logged in member is an Administrator, they can select a member to impersonate.

    public IActionResult Impersonate([FromQuery]string memberId)
    {
        //Get the member to impersonate
        var member = _memberService.GetById(Convert.ToInt32(memberId));
        var signinuser = _memberManager.FindByNameAsync(member.Username).Result;

        //Get the currently logged in
        var currentUser = _memberManager.GetCurrentMemberAsync().Result;
        var currMember = _memberService.GetByKey(currentUser.Key);

        //signout currentuser
        _signInManager.SignOutAsync();

        //Sign in as impersonated user
        _signInManager.SignInAsync(signinuser,false);
        HttpContext.Session.SetString("portalImpersonate", currentUser.Id);

        return CurrentUmbracoPage();

    }