Add static rows in dynamic mat-table

I use mat-table and would like to insert two static lines. In these static lines input is made and later stored in the back-end. Can you please tell me how to insert the static rows and make them available for processing?

My Code:

// HTML
<td mat-cell class="table-content-bg-second" id="table-content-font-second" *matCellDef="let row; let i = index;">
    <div formArrayName="rows" *ngIf="attributesWithFormControls.includes(column.attribute); else 
      otherColumns">
                  <span class="edit-cell" [formGroupName]="i">
                      <mat-form-field>
                        <label>
                          <input matInput type="text" [formControlName]="column.attribute">
                        </label>
                      </mat-form-field>
                    </span>
                </div>
                <ng-template #otherColumns>
                  {{ column.object  !== null ? row[column.object][column.attribute] : row[column.attribute]  }}
                </ng-template>
              </td>
// TS
private monthColumns: EditColumns[] = [
    {attribute: '1', name: 'Jan', object: 'values', disabledRanges: false, disabledAssignment: false},
    {attribute: '2', name: 'Feb', object: 'values', disabledRanges: false, disabledAssignment: false},
    {attribute: '3', name: 'Mrz', object: 'values', disabledRanges: false, disabledAssignment: false},
    {attribute: '4', name: 'Apr', object: 'values', disabledRanges: false, disabledAssignment: false},
  ];

  private afterMonthsColumns: EditColumns[] = [
    {attribute: 'sum', name: 'Summe', object: 'values', disabledRanges: true, disabledAssignment: false},
    {attribute: 'note', name: 'Anmerkung', object: null, disabledRanges: false, disabledAssignment: false},
  ];

  // Merged arrays
  public displayedColumns: EditColumns[] = [
    ...this.monthColumns,
    ...this.afterMonthsColumns
  ];

Source: Angular Material Quesions