How to change bitrate with nreco functions using textbox?

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C#

Visual-Studio

video

I'm trying to write a converter in C # using Nreco.VideoConverter. I've never had any experience with these before. I started researching this area because I was asked at work. My problem is; I can change the bitrate value in video converter. If you have a combobox or a specific value. But, if there is any value to enter from the textbox, I cannot adapt the code accordingly. Below is the code I use. Please help.

Code is;
if (comboBox2.Text == "_1000kbit")
{
if (comboBox1.Text == "mp4" || comboBox1.Text == "mp4 1280 x 720 16 : 9" || comboBox1.Text == "mp4 640 x 350 16 : 9" || comboBox1.Text == "mp4 720 x 540")
{
var ffmpeg = new NReco.VideoConverter.FFMpegConverter();
ffmpeg.ConvertMedia(VideoPath, null, MusicPath, null, new ConvertSettings()
{
CustomOutputArgs = "-b:v 1000k -bufsize 1000k"
});
}

}


but ı want do this;

if (comboBox2.Text == "_1000kbit")
{
if (comboBox1.Text == "mp4" || comboBox1.Text == "mp4 1280 x 720 16 : 9" || comboBox1.Text == "mp4 640 x 350 16 : 9" || comboBox1.Text == "mp4 720 x 540")
{
var ffmpeg = new NReco.VideoConverter.FFMpegConverter();
ffmpeg.ConvertMedia(VideoPath, null, MusicPath, null, new ConvertSettings()
{
CustomOutputArgs = "-b:v"+textBox1.Text+"k -bufsize"+textBox1.Text+"k"
});
}

}

So is it possible? how can i do it if possible?
Because when I type it it says ffmpeg can't find the argument.
By the way, I set the textbox value to int.
Please help for this.
Thank You.

What I have tried:

ı want do this; but is it possible or true I have no idea

if (comboBox2.Text == "_1000kbit")
{
if (comboBox1.Text == "mp4" || comboBox1.Text == "mp4 1280 x 720 16 : 9" || comboBox1.Text == "mp4 640 x 350 16 : 9" || comboBox1.Text == "mp4 720 x 540")
{
var ffmpeg = new NReco.VideoConverter.FFMpegConverter();
ffmpeg.ConvertMedia(VideoPath, null, MusicPath, null, new ConvertSettings()
{
CustomOutputArgs = "-b:v"+textBox1.Text+"k -bufsize"+textBox1.Text+"k"
});
}

}