143补材料 Undertaking from your sponsor in relation to all applicants

as6400

父母143申请最近收到补材料通知。其中一项好像别人都没有：

Undertaking from your sponsor in relation to all applicants
Provide a letter from your sponsor agreeing to extend their sponsorship undertakings to
cover all applicants included in this visa application (for example, your spouse or children).
The letter should include the:
● name of sponsor
● full name and date of birth of primary visa applicant
● name and date of birth of each secondary visa applicant included in the application.

请问这个应该怎么写，自己写一封信吗，有没有措辞和格式的要求？

另外好像没看到其他143的申请收过这个，是我漏了什么东西吗？请有经验的大神帮忙，先谢过了