Is it necessary to add a @ in front of an SqlParameter name?

Ref: SqlParameter.ParameterName Property and IDataParameter.ParameterName Property

The ParameterName is specified in the form @paramname. You must set ParameterName before executing a SqlCommand that relies on parameters. If you are using Sql Server as Database then you must specify @ before the parameter name.

your parameter name must be same as at backend eg. you have @searchText then in your parameter specification it must be SqlParameter("@searchText" ..

your code should be like this

Dim parm As New SqlParameter("@searchText", SqlDbType.VarChar)
parm.Direction = ParameterDirection.Input
parm.Size = 50
parm.Value="test"

cmd.Parameters.Add(parm)

Note: Oracle and SqLite use different use different character to specify parameter and there may be @ symbol is not used specified by the specification of ado.net.

Edit: By comments

As you specified the link, it is also some sort of fix, but as per the msdn documentation, you must specify the positional parameter with '@' whether you are using any data provider oledb, sql, odbc. Ref

if (0 < parameterName.get_Length() && '@' != parameterName.get_Chars(0))
        {
            parameterName = "@" + parameterName;
        }

According to the documentation, the name must start with an @:

The ParameterName is specified in the form @paramname.

According to the source code (have a look at SqlCommand and SqlParameter.ParameterNameFixed in the reference source), an @ is added automatically, if needed.

So yes, it works, but it's an undocumented feature. Best practice recommends that you do not rely on this and manually prefix your parameter name with an @.