First you should select the right selector to extract script tag information as text.

text = url.xpath('//body/script/text()').get()

Then you can use regex to find what you want.

import re
mp4 = re.compile(r"(?<=mp4:\s\[')(.*)'\]")
print(mp4.findall(text)[0])

See @CypherX for same result with beautifullsoup.

Output

https://mvd8.ddns.me:443/viewm/88/686/88686.mp4?wmsAuthSign=c2VydmVyX3RpbWU9MTAvMjMvMjAxOSAzOjMwOjE3IFBNJmhhc2hfdmFsdWU9UXgrZ1dHTWxhVGdNM0Iyd3dSeHJBdz09JnZhbGlkbWludXRlcz02MA==

Data

text = """
<script>
    var bitrates = {
         mp4: ['https://mvd8.ddns.me:443/viewm/88/686/88686.mp4?wmsAuthSign=c2VydmVyX3RpbWU9MTAvMjMvMjAxOSAzOjMwOjE3IFBNJmhhc2hfdmFsdWU9UXgrZ1dHTWxhVGdNM0Iyd3dSeHJBdz09JnZhbGlkbWludXRlcz02MA==']
    };

        var schedule = {
        preroll: [
            'https://googleads.g.doubleclick.net/pagead/ads?ad_type=video_text_image&client=ca-video-pub-1231661633440980&description_url=https%3A%2F%2Fwww.farfeshplus.com&channel=7962520214&videoad_start_delay=0&hl=ar'
            ],
        midroll: [

            [600,'https://googleads.g.doubleclick.net/pagead/ads?ad_type=video_text_image&client=ca-video-pub-1231661633440980&description_url=https%3A%2F%2Fwww.farfeshplus.com&channel=7962520214&videoad_start_delay=0&hl=ar'],

            [1200,'https://pubads.g.doubleclick.net/gampad/ads?iu=/60345044/Pirsom_Ayoub_LTD_TOP/farfeshplus/farfeshplus_Preroll&description_url=https%3A%2F%2Fwww.farfeshplus.com%2F&env=vp&impl=s&correlator=&tfcd=0&npa=0&gdfp_req=1&output=vast&sz=640x480&unviewed_position_start=1'],

            [1800,'https://googleads.g.doubleclick.net/pagead/ads?ad_type=video_text_image&client=ca-video-pub-1231661633440980&description_url=https%3A%2F%2Fwww.farfeshplus.com&channel=7962520214&videoad_start_delay=0&hl=ar']
            ],
        postroll: [
          'https://pubads.g.doubleclick.net/gampad/ads?iu=/60345044/Pirsom_Ayoub_LTD_TOP/farfeshplus/farfeshplus_Preroll&description_url=https%3A%2F%2Fwww.farfeshplus.com%2F&env=vp&impl=s&correlator=&tfcd=0&npa=0&gdfp_req=1&output=vast&sz=640x480&unviewed_position_start=1'
          ]
    };
        var settings = {
        licenseKey: 'Kl8lNHNrNzkyY3M5dj9yb201ZGFzaXMzMGRiMEElXyo=',
        bitrates: bitrates,
        delayToFade: 3000,
        width: 750,
        height: 440,
        skin: 's4',
        poster: 'https://images.farfeshplus.com/videos/lrg/laila_m_29.jpg',
        ads: true,      
        adSchedule: schedule
    };
    var elementID = 'rmpPlayer';
    var rmp = new RadiantMP(elementID);
    rmp.init(settings);
</script>
"""

Another option is to use BeautifulSoup along with regex. The regex part is the same as suggested by @FlorianBernard.

from bs4 import BeautifulSoup
import re

soup = BeautifulSoup(text, "html.parser")
script = soup.find_all('script')[1]
mp4 = re.compile(r"(?<=mp4:\s\[\')(.*)\'\]")
print(mp4.findall(script.get_text())[0])

Output:

https://mvd8.ddns.me:443/viewm/52/653/52653.mp4?wmsAuthSign=c2VydmVyX3RpbWU9MTAvMjMvMjAxOSA2OjI2OjAzIFBNJmhhc2hfdmFsdWU9ODlyM3FWTlRONldQWGJOT3JWQWJTUT09JnZhbGlkbWludXRlcz02MA==

Data

Here text is the variable that contains the entire html document.

text = """
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>RikTak Video Player - Version 1</title>
    <script src="https://cdn.radiantmediatechs.com/rmp/5.2.1/js/rmp.min.js"></script>
    <style>
        body {
            margin: 0;
        }
    </style>
</head>
<body>
<div id="rmpPlayer"></div>
<script>
    var bitrates = {
         mp4: ['https://mvd8.ddns.me:443/viewm/52/653/52653.mp4?wmsAuthSign=c2VydmVyX3RpbWU9MTAvMjMvMjAxOSA2OjI2OjAzIFBNJmhhc2hfdmFsdWU9ODlyM3FWTlRONldQWGJOT3JWQWJTUT09JnZhbGlkbWludXRlcz02MA==']
    };

        var schedule = {
        preroll: [
            'https://googleads.g.doubleclick.net/pagead/ads?ad_type=video_text_image&client=ca-video-pub-1231661633440980&description_url=https%3A%2F%2Fwww.farfeshplus.com&channel=7962520214&videoad_start_delay=0&hl=ar'
            ],
        midroll: [

            [600,'https://googleads.g.doubleclick.net/pagead/ads?ad_type=video_text_image&client=ca-video-pub-1231661633440980&description_url=https%3A%2F%2Fwww.farfeshplus.com&channel=7962520214&videoad_start_delay=0&hl=ar'],

            [1200,'https://pubads.g.doubleclick.net/gampad/ads?iu=/60345044/Pirsom_Ayoub_LTD_TOP/farfeshplus/farfeshplus_Preroll&description_url=https%3A%2F%2Fwww.farfeshplus.com%2F&env=vp&impl=s&correlator=&tfcd=0&npa=0&gdfp_req=1&output=vast&sz=640x480&unviewed_position_start=1'],

            [1800,'https://googleads.g.doubleclick.net/pagead/ads?ad_type=video_text_image&client=ca-video-pub-1231661633440980&description_url=https%3A%2F%2Fwww.farfeshplus.com&channel=7962520214&videoad_start_delay=0&hl=ar']
            ],
        postroll: [
          'https://pubads.g.doubleclick.net/gampad/ads?iu=/60345044/Pirsom_Ayoub_LTD_TOP/farfeshplus/farfeshplus_Preroll&description_url=https%3A%2F%2Fwww.farfeshplus.com%2F&env=vp&impl=s&correlator=&tfcd=0&npa=0&gdfp_req=1&output=vast&sz=640x480&unviewed_position_start=1'
          ]
    };
        var settings = {
        licenseKey: 'Kl8lNHNrNzkyY3M5dj9yb201ZGFzaXMzMGRiMEElXyo=',
        bitrates: bitrates,
        delayToFade: 3000,
        width: 750,
        height: 440,
        skin: 's4',
        poster: 'https://images.farfeshplus.com/videos/lrg/laila_m_29.jpg',
        ads: true,      
        adSchedule: schedule
    };
    var elementID = 'rmpPlayer';
    var rmp = new RadiantMP(elementID);
    rmp.init(settings);
</script>
</body>
</html>
"""