The platform has recently introduced a feature called "app tagging" that allows you to enable a custom MDA to run in the FS App container. A UI for app tagging will become available in 2021, in the meantime use the following workaround:
To target your App Modules for the Field Service Mobile application.
1. Create & Publish your custom App Module. In this example we’ll call it “ContosoAppModule”
2. Associate your app module to a Solution.
3. Export the Solution
4. Open the exported Solution (.zip) and edit the file “Customizations.xml”
5. Locate XML node for <AppModule>
6. Within <AppModule> insert a section element <appsettings> as shown below, where:
a. App settings uniquename is the name of your new app module. This follows format of [publisher prefix]_[app module unique name]_[app settings unique name]. Or “new_new_ContosoAppModule_ChannelAvailability” in our example.
b. value is the application you wish to target, “FieldServiceMobile”. This can be a comma delineated list if you want to target multiple apps.
7. Save the updated XML file and re-add to the .zip file of your solution.
8. Import the file back into your Dynamics organization.
<appsettings>
<appsetting uniquename="Your Unique Name">
<value>FieldServiceMobile</value>
<iscustomizable>1</iscustomizable>
<displayname default="App channel availability">
<label description="App channel availability" languagecode="1033" />
</displayname>
<description default="Specifies the channel in which the given app would be available.">
<label description="Specifies the channel in which the given app would be available." languagecode="1033" />
</description>
<settingdefinitionid>
<uniquename>ChannelAvailability</uniquename>
</settingdefinitionid>
</appsetting>
</appsettings>
