为了实现水平对齐，你需要在标记中做一些调整，并使用flex属性来实现完美的对齐。我添加了一些div作为父类，但如果你愿意，你可以将CSS分配给你代码的实际父类。我在下面添加了我的代码片段，相应地更新HTML和CSS：

.searchcontainer {
          text-align:center;
          width: 100%;
          max-width: 600px;
          margin: 0 auto;
       }

       .searchcontainer .search-part {
          display: flex;
          flex-direction: row;
          justify-content: center;
          align-items: center;
       }

      mat-form-field.mat-form-field {
          font-size: 12px;
          width: 300px;
      }

      button.mat-stroked-button {
          font-size: 14px;
          height: 42px;
          margin: 10px;
      }

<div class="searchcontainer">

      <p>Search by company name:</p>

      <div class="search-part">

        <mat-form-field appearance="outline"> 
            <mat-label>Search</mat-label>
            <input matInput placeholder="Search">
        </mat-form-field>

        <button mat-stroked-button color="primary">Search</button>
      </div>

    </div>

<mat-form-field>
  <mat-label>Type Postcode</mat-label>
  <input matInput placeholder="Postcode" formControlName="postCode" required>
  <button matSuffix mat-button (click)=""><mat-icon >search</mat-icon></button>
</mat-form-field>