spfx webparts – Get selected value of fabric dropdown

hi All i want to get value of selected dropdown in my Visacontact function , below is my code

  <Fabric  className={styles.matrix2}>
    <VirtualizedComboBox
      styles={comboBoxStyles}
      allowFreeform
      autoComplete="on"
      options={this.state.ddlbookcountry}
      dropdownMaxWidth={200}
      useComboBoxAsMenuWidth
      defaultValue="Please select Key"
     
      
    />
  </Fabric>
< input className={styles.button} type='Button' value="Visa Contact" onClick={()=>{this.VisaContact()}}>     
      </input>