Indentation for c# functions
There are two places where I would use var:
var conn = new MySqlConnection(ConnectionString.ConnString)
var complaint = new List&lt;ComplaintModel&gt;();

You no longer have to put braces around a using block, saving you one indentation level:
using var conn = new MySqlConnection(ConnectionString.ConnString);

Don't put the first parameter at the end of the line and the rest on a new line. Put the first parameter on a new line as well:
complaint.Add(new ComplaintModel {
    Date = _Date,
    ...

If you start the Query string on a new line, you can move it back one indentation level:
conn.Execute(
    @&quot;INSERT INTO customer_complaints
    ...