open interface B in interface A in portal

Hi All,

I have a Signup form on the appian portals.

case 1: I want when I click on the Login then Open the Login UI.

Interface A: 

And when the user enter all the details in the Signup form once and submit the form all the details are save in DB.

case 2: I want if the same user again come in Portal and signup form fill then show a message "you already have an account please login". In that case how to check the details of user in DB is present or not and show the message?

Interface B: 
a!localVariables(
  local!accountDetails: a!map(firstName: null, lastName: null, email: null,Password: null),

  a!sectionLayout(
    contents: {
      a!columnsLayout(
        columns: {
          a!columnLayout(contents: {}),
          a!columnLayout(contents: {}),
          a!columnLayout(
            contents: {
              a!cardLayout(
                contents: {
                  a!cardLayout(
                    /* Create Account */
                    contents: {
                      a!richTextDisplayField(
                        labelPosition: "COLLAPSED",
                        value: {
                          a!richTextItem(
                            text: "Create account",
                            color: "#434343",
                            size: "LARGE",
                            style: "STRONG"
                          )
                        },
                        marginBelow: "STANDARD"
                      ),
                      a!richTextDisplayField(
                        labelPosition: "COLLAPSED",
                        value: {
                          a!richTextItem(
                            text: "Already have an account?",
                            color: "#666666"
                          ),
                          " ",
                          a!richTextItem(
                            text: {
                              a!richTextItem(
                                text: "Log in",
                                color: "ACCENT"
                              )
                            },
                            link:
                            a!dynamicLink(
                              value:rule!TestLoginUI(),
                            )
                          
                          )
                        },
                        marginBelow: "MORE"
                      ),
                      a!textField(
                        label: "First Name",
                        value:  local!accountDetails.firstname,
                        saveInto: local!accountDetails.firstname
                      ),
                      a!textField(
                        label: "Last Name",
                        value:  local!accountDetails.lastname,
                        saveInto: local!accountDetails.lastname
                      ),
                      a!textField(
                        label: "Email",
                        value:  local!accountDetails.email,
                        saveInto: local!accountDetails.email
                      ),
                      a!textField(
                        label: "Password",
                        value:  local!accountDetails.Password,
                        saveInto: local!accountDetails.Password,
                      ),
                      a!buttonArrayLayout(
                        buttons: {
                          a!buttonWidget(
                            label: "Signup account",
                            width: "FILL",
                            style: "SOLID"
                          )
                        },
                        align: "START",
                        marginAbove: "MORE"
                      )
                    },
                    shape: "ROUNDED",
                    padding: "MORE",
                    marginBelow: "STANDARD",
                    showBorder: false,
                    showShadow: true
                  )
                },
                /*style: local!backgroundColor,*/
                padding: "EVEN_MORE",
                showBorder: false
              )
            },
            width: "WIDE"
          ),
          a!columnLayout(contents: {}),
          a!columnLayout(contents: {})
        },
        alignVertical: "MIDDLE",
        stackWhen: { "PHONE", "TABLET_PORTRAIT" }
      )
    },
 
  ),
 
  
)


Please help me for both the cases

Thanks