How to get a list of Process models Id in an application?

You can get it through a report. Create an 'All Process Models' report. In that report, add a process model ID column with the definition "pm!id"

Hi, umarm7858 , 
Agree to what Yogi Patel  mentioned, 
In addition to that, 
Just go onto the Process Report, click on magnifier icon at the end for applying filter.(as highlighted in the yellow color), 
Then you can see the text box below to type the Application Prefix (as highlighted in the green color). 
Another way is to use this report in a!queryProcessAnalytics and use that to fetch data on UI. (apply filters)
 

Note- Here is a consideration, that your application's prefix is used in PM name. 

Beware that it can also fetch the PMs which have this prefix in their name as mentioned by Stefan.
If that's the case, you need to apply additional text filtration over the field to only get the names which start with your Prefix, and ignore the ones which have prefix in between the name. 

For this to work, you need to apply some first level of filter inside a!query() of aa!queryProcessAnalytics() which is used in your UI, and an additional ER to filter this data to get the names of PM, only starting with the prefix.
I haven't tried out the UI method, but you can try and let us know if that works. 

Hope this helps.  

Thanks, although i made some adjustment but it worked well.

Thanks, although i made some adjustment but it worked well.