c# Using Parameters.AddWithValue in SqlDataAdapter

The string used to initialize the SqlDataAdapter becomes the CommandText of the SelectCommand property of the SqlDataAdapter.
You could add parameters to that command with this code

da = new SqlDataAdapter("SELECT * FROM annotations WHERE annotation LIKE @search",
                        _mssqlCon.connection);
da.SelectCommand.Parameters.AddWithValue("@search","%" + txtSearch.Text + "%");

• First, remove the single quote around the parameter placeholder.
• Second, add the wildcard character directly in the Value parameter of
  AddWithValue

You have asked to use AddWithValue, but remember that, while it is a useful shortcut, there are also numerous drawbacks and all well documented.

• First: Can we stop using AddWithValue() already? where the
  author discuss how AddWithValue could give back wrong results in your
  queries
• Second: How Data Access Code Affects Database Performance where
  the author presents evidences of strong performance problems for
  AddWithValue

So, the same code without AddWithValue and using the Object and Collection Initializers syntax could be written as

da = new SqlDataAdapter("SELECT * FROM annotations WHERE annotation LIKE @search",
                        _mssqlCon.connection);
da.SelectCommand.Parameters.Add(new SqlParameter
{
    ParameterName = "@search",
    Value = "%" + txtSearch.Text + "%",
    SqlDbType = SqlDbType.NVarChar,
    Size = 2000  // Assuming a 2000 char size of the field annotation (-1 for MAX)
});

and, an even more simplified and one liner version of the above is:

da.SelectCommand.Parameters.Add("@search",SqlDbType.NVarChar,2000).Value = "%" + txtSearch.Text + "%";