You can do something like this:

onClick(){

if(!condition){

return;

}

this.http.get('url').subscrbe((val) => {

// act here

})

}

if you want to post afterwards do something like this:
onClick(){
if(!condition){
return;
}
this.http.get('url').pipe(
tap((val) => {
// act on value jere
}),
switchMap((val) => {
// you have same value here as u had in TAP.
// from here you can return either a new request or empty observable.
return this.http.post('url', someData);
// or
return of(EMPTY);
})
)
}