how to post ngx-select-dropdown to web api

Question

I'm using ngx-select-dropdown then we will get the data from the database.

HTML

                                 <td class="dropdown">
                                    <ngx-select-dropdown 
                                        [config]="employeConfig"
                                        [options]="responseDS"
                                        [value]="responseDS"
                                        formControlName="office">
                                    </ngx-select-dropdown>
                                 </td>

.TS

         this.employeConfig = {
         displayKey: 'officeDesc',   // NAME OF THE COLUMN IN DATABASE
         search: true,
         placeholder: '',
         limitTo: this._Obj.length,
         moreText: 'officeDesc',
         noResultsFound: 'No results found',
         searchPlaceholder: 'Search',
         }

How will I pass the selected drop-down to the WebAPI?