card layout- make as required

Hi all, 

I have the below requirement in which I have a card layout with 3 options, I wanted to make it as a mandatory field, Can someone please help here how can we make this as required?

Here when the user clicks the submit button, there should be a validation and wanted that field to be a mandatory one. 

a!sectionLayout(
  label: "",
  contents: {
    a!columnsLayout(
      columns: {
        a!columnLayout(
          contents: {
            a!richTextDisplayField(
              labelPosition: "COLLAPSED",
              value: {
                a!richTextItem(
                  text: {
                    a!richTextIcon(icon: "folder-open"),
                    " Type"
                  },
                  size: "MEDIUM_PLUS",
                  style: { "STRONG" }
                ),
                
              }
            )
          },
          width: "AUTO"
        ),
        a!columnLayout(
          contents: {
            a!richTextDisplayField(
              labelPosition: "COLLAPSED",
              value: {
                a!richTextItem(text: { "Category" }, style: { "STRONG" })
              }
            ),
            a!columnsLayout(
              columns: {
                a!columnLayout(
                  contents: {
                    a!cardLayout(
                      contents: {
                        a!richTextDisplayField(
                          labelPosition: "COLLAPSED",
                          value: {
                            a!richTextIcon(
                              icon: "money",
                              color: "ACCENT",
                              size: "LARGE_PLUS"
                            ),
                            char(10),
                            a!richTextItem(
                              text: { "Fundraiser" },
                              color: "SECONDARY",
                              size: "STANDARD"
                            ),
                            char(10),
                            char(10)
                          },
                          align: "CENTER"
                        )
                      },
                      height: "AUTO",
                      style: "NONE",
                      padding: "LESS",
                      marginBelow: "STANDARD"
                    )
                  }
                ),
                a!columnLayout(
                  contents: {
                    a!cardLayout(
                      contents: {
                        a!richTextDisplayField(
                          labelPosition: "COLLAPSED",
                          value: {
                            a!richTextIcon(
                              icon: "bullhorn",
                              color: "ACCENT",
                              size: "LARGE_PLUS"
                            ),
                            char(10),
                            a!richTextItem(
                              text: { "Awareness" },
                              color: "SECONDARY",
                              size: "STANDARD"
                            ),
                            char(10),
                            char(10)
                          },
                          align: "CENTER"
                        )
                      },
                      link: a!dynamicLink(label: "Dynamic Link", saveInto: {}),
                      height: "AUTO",
                      style: "NONE",
                      padding: "LESS",
                      marginBelow: "STANDARD"
                    )
                  }
                ),
                a!columnLayout(
                  contents: {
                    a!cardLayout(
                      contents: {
                        a!richTextDisplayField(
                          labelPosition: "COLLAPSED",
                          value: {
                            a!richTextIcon(
                              icon: "handshake-o",
                              color: "ACCENT",
                              size: "LARGE_PLUS"
                            ),
                            char(10),
                            a!richTextItem(
                              text: { "Lobbying" },
                              color: "SECONDARY",
                              size: "STANDARD"
                            ),
                            char(10),
                            char(10)
                          },
                          align: "CENTER"
                        )
                      },
                      link: a!dynamicLink(label: "Dynamic Link", saveInto: {}),
                      height: "AUTO",
                      style: "NONE",
                      padding: "LESS",
                      marginBelow: "STANDARD"
                    )
                  }
                )
              }
            ),
            
          }
        )
      },
      spacing: "SPARSE"
    )
  },
  divider: "ABOVE"
),