To do what you are after you could do something like this (warning this hasn't been tested!)
string ValueEnteredByTextBox = "[ja+emsak]";
string SQL = "SELECT * FROM sometable where somecolumn like @Param";
string LikeClause = ValueEnteredByTextBox.replace("+","%");
using(SqlConnection cn = new SqlConnection(DatabaseConnectionString))
{
cn.open();
SqlCommand cmd = new SqlCommand(sql,cn);
cmd.Parameters.AddWithValue("@Param", LikeClause);
}