思路1：输入信息的时候，给信息加入{!}的字符

全局代码

Public Sub textbox_TextChanged(sender As object, e As EventArgs)
If sender.text.startswith("{!}") = False Then
    sender.text = "{!}" & sender.text
    sender.SelectionStart = sender.text.length
    sender.SelectionLength = 1
End If
End Sub

打开聊天窗口代码

QQClient.OpenChatWindow()
Dim frm As windows.forms.form = Application.OpenForms("frmTalker")
If frm IsNot Nothing Then
    Dim obj = frm.controls("SplitContainer1")
    Dim edit As windows.forms.textbox = obj.panel2.controls("C1DockingTab2").controls("C1DockingTabPage3").controls("SplitContainer2").panel2.controls("txtSender")
    Addhandler edit.textchanged, addressof textbox_textchanged
End If

思路2：类似思路1，在聊天窗口加入一个按钮【弹出发送】，然后绑定click事件，当点击按钮的时候，给文本框加上{!}的字符，然后触发发送按钮。

思路3：类似思路1，给文本框绑定keydown事件，如果触发 alt+enter，就弹出发送。

全局代码

Public Sub textBox1_KeyDown(sender As Object, e As System.Windows.Forms.KeyEventArgs)
If e.alt AndAlso e.KeyCode = Keys.Enter Then
    If sender.text.startswith("{!}") = False Then
        sender.text = "{!}" & sender.text
        sender.parent.Controls("btnSend").PerformClick
    End If
End If
End Sub

打开聊天窗口

QQClient.OpenChatWindow()
Dim frm As windows.forms.form = Application.OpenForms("frmTalker")
If frm IsNot Nothing Then
    Dim obj = frm.controls("SplitContainer1")
    Dim edit As windows.forms.textbox = obj.panel2.controls("C1DockingTab2").controls("C1DockingTabPage3").controls("SplitContainer2").panel2.controls("txtSender")
    Addhandler edit.keydown, addressof textBox1_KeyDown
End If

好的感谢甜老师