Initialize nested struct definition

Your Contact is a field with anonymous struct type. As such, you have to repeat the type definition:

s := &Sender{
    BankCode: "BC",
    Name:     "NAME",
    Contact: struct {
        Name  string
        Phone string
    }{
        Name:  "NAME",
        Phone: "PHONE",
    },
}

But in most cases it's better to define a separate type as rob74 proposed.

How about defining the two structs separately and then embedding "Contact" in "Sender"?

type Sender struct {
    BankCode string
    Name     string
    Contact
}

type Contact struct {
    Name  string
    Phone string
}

if you do it this way, your second initialization attempt would work. Additionally, you could use "Contact" on its own.

If you really want to use the nested struct, you can use Ainar-G's answer, but this version isn't pretty (and it gets even uglier if the structs are deeply nested, as shown here), so I wouldn't do that if it can be avoided.