Static Tab with placeholder values not displayed - json

Question

I am currently getting into Microsoft Team Apps and am trying to create a very simple tool.
All it should be is a static tab, that when visited embeds a certain website, that's given the groupId as a parameter. This is the format, that I have in mind for the contentUrl: https://foo.bar?groupId=\{groupId\}
The app consists only of two images, that are PNGs in 20x20px (transparent) and 96x96px (with background color) and the manifest (version 1.2).
The manifest contains all required fields:
$schema, manifestVersion, version, id, packageName, developer {name, websiteUrl, privacyUrl, termsOfUseUrl}, icons {color, outline}, description {short, full}, accentColor, permissions, validDomains, staticTabs [{entityId, name, contentUrl, scopes}]
An Id for the app has been generated at apps.dev.microsoft.com.
The upload of the custom app as zip file causes no error and I can see the app in the list of the installed applications. If I switch to a channel I can't see the tab which should be there without being explicitly added because it's a static tab. Adding it manually isn't possible because it's not shown in the list of "addable apps".
I tried some of the example apps from GitHub, but they seem outdated and at least partly can't be even read (Error while reading manifest.json).
Is this sufficient information to help me?
Edit: (anonymized manifest.json)
{
"$schema":
"https://statics.teams.microsoft.com/sdk/v1.2/manifest/MicrosoftTeams.schema.json",
"manifestVersion": "1.2",
"version": "1.0.0",
"id": "12345678-abcd-efgh-1234-123456789012",
"packageName": "com.microsoft.teams.bar",
"developer": {
"name": "Foo Corp",
"websiteUrl": "https://foo.com",
"privacyUrl": "https://foo.com/privacy",
"termsOfUseUrl": "https://foo.com/termsofuse"
},
"icons": {
"color": "src/images/waldo-96x96.png",
"outline": "src/images/waldo-20x20.png"
},
"name": {
"short": "bar",
"full": "bar"
},
"description": {
"short": "Opens baz and does qux.",
"full": "Opens baz and does qux."
},
"accentColor": "#FFFFFF",
"permissions": ["identity", "messageTeamMembers"],
"validDomains": ["*.baz.com", "*.foo.com"],
"staticTabs": [
{
"entityId": "tabId",
"name": "example tab",
"contentUrl":
"https://foo.com/?teamId={teamId}&teamName={teamName}&channelId={channelId}&channelName={channelName}&locale={locale}&theme={theme}&entityId={entityId}&subEntityId={subEntityId}&upn={upn}&tid={tid}&groupId={groupId}&theme={theme}&isFullScreen={isFullScreen}",
"scopes": ["personal"]
}
]
}

Answer 1

I could see two issues in your manifest code:
Invalid GUID: 12345678-abcd-efgh-1234-123456789012 is not valid GUID.
Icons must be included in the package and referenced via relative paths in the manifest.
ex. waldo-96x96.png - Please place them inside zip file.
Please feel free to use Teams App Studio to create the manifest file. You can also use the existing form the sample code.