Since you have a value in a text box that you want to use as the timer interval, you need to convert that textbox value to an integer before assigning it to the timer value. For example, you could do it like this:
TextBox1.Text = RandX.Next(195, 205);
int timerNumber = Convert.ToInt32(TextBox1.Text);
yourTimer.Interval = timerNumber;
Update
Based upon the comments below, this is how your code should look:
Private Sub Timer1_Tick(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Timer1.Tick
Dim RandX As New Random
TextBox1.Text = RandX.Next(195, 205)
Dim timerNumber As Integer = Convert.ToInt32(TextBox1.Text)
Timer2.Interval = timerNumber
End Sub
Private Sub Timer2_Tick(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Timer2.Tick
mouse_event(mouseclickdown, 0, 0, 0, 0)
mouse_event(mouseclickup, 0, 0, 0, 0)
End Sub
The one thing to pay attention to, though, is that Timer2 is assumed to already be running. That might not be what you want. If not, you should probably stop Timer2 after it runs and start it once you set the interval in Timer1. Something like this:
Private Sub Timer1_Tick(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Timer1.Tick
Dim RandX As New Random
TextBox1.Text = RandX.Next(195, 205)
Dim timerNumber As Integer = Convert.ToInt32(TextBox1.Text)
Timer2.Interval = timerNumber
Timer2.Start()
End Sub
Private Sub Timer2_Tick(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Timer2.Tick
mouse_event(mouseclickdown, 0, 0, 0, 0)
mouse_event(mouseclickup, 0, 0, 0, 0)
Timer2.Stop()
End Sub